Pardon me for paraphrasing, but it sounds like what you are asking is "Did the history of Africa have any affect on its current state?" Which to any historian the answer is always going to be yes.
Now if you want to know "how much" of the issues that African countries currently face are due to colonializm, that is going to be hard to quantify. You have to remember Africa is huge, 30 million square miles and over a billion people. It has (now) 54 different countries. Many more tribal groups and languages. Clearly European influence has drastically impacted all the countries in Africa, but it is impossible to say that is responsible for all instability on the continent. It had different effects in different places. So while you can say that European influence led to Africa being the way it is, that isn't the only factor. The current state is also because of war, famine, tribal and religious tensions, the Cold War, military coups, and global ecconomic conditions. All of which had different effects in different places at different times. In short there is no ways to say something like "78% of instability in Africa is due to colonialism."
I hope I am not discouraging you in any way. The history of Africa is just as fascinating as European, Asian, or American history is. Too few people know anything about it. So I highly recommend you look into it further. The issue is so nuanced though it just can't be painted with a single broad brush.
A large degree of it. European interference, ongoing and historical, did damage across a large swathe of cultures and institutions. African governments were destroyed, agricultural traditions were supplanted to satisfy European import requirements, peoples were displaced, borders were drawn without proper input from the people they were drawn around. Europeans had their own ideas about how to run things, and when the ideas weren't based in outright incorrect assumptions about human nature or horrible science, they were based in racist ideologies and the capitalist drive to profit. Unfortunately, even when the intent is to be helpful, they still got it wrong on many occasions, doing more harm than good. For example, as discussed by Fairhead and Leach in Misreading the African landscape: Society and Ecology in a Forest-Savanna Mosaic, it was found that Colonial managment of the Savannah-Forest mosaic in western Africa, with the goal of preservation, ended up doing more harm than good, having the opposite intended effect. By ignoring established local knowledge and imposing European rule, the colonial government and its sociologists completely missed that it was the local population which had created the forests they were trying to preserve (it was their assumption that the locals were having a negative effect on the environment), and so instead the colonial powers worsened the situation by preventing the local peoples from exercising their cultivation practices which had created the forests in the first place. This had the additional effect of severely limiting the people's agency and potential for development. Additional to their assumption that the human populations were a recent and negative addition to the landscape was their assumption that these peoples had always been poor. Further research found that they had actually only become poor with the introduction of European management. This is just one small case study out of many, and it involves government actions that are continuing to this day. European imperialism is far from over, research about the relationship between the west and the developing world shows that through the use of finance and highly inuequal trade relations, the west extracts several trillion dollars in wealth every year from the developing world as a whole, with much of this coming from Africa in the form of natural resources (and soon, labor).
I would recommend reading this to get a good introduction to the topic.
Economist here. This is more of a social science question and I may chime in. Yes you can quantify and give an answer which is of the nature of 'on average'. One of the most influential papers in the recent past was from Acemoglu, Johnson & Robinson (2001) (AJR hereafter) which is an empirical paper to determine the causes of the differences of economic development between the colonies. For AJR, institutions are a significant cause of economic development. During the colonization, one factor which determined institution is settler mortality which correlated with whether having a good or bad institution. Simply: where settler mortality was high, colonizers did not settle but rather installed exploitative institutions. Now bad institutions are hard to get rid of, even after the end of the colonial powers reign its fundamentals persist within the decolonized government. So settler mortality determined the colonizers choice of whether to install an exploitative institution or non-exploitative, where settler could settle to. In fact, AJR empirical analysis lead to the conclusion that institutions are significant explanatory variable which determines economic development, and that other factors such as geography is not significant (anymore). The paper is a little more technical than my explanation. If you understand a little statistics/econometrics: They used settler mortality as an instrumental variable to address the endogeneity problem (mostly measurement error) with the proxy variable for institutions "protection against appropriation risk" to explain cross country variances in colony GDP per capita.
So to answer your question: European colonization influenced African instability mostly due to installing exploitative institutions in the past because settler mortality was high. Where there is low settler mortality, 'normal' institutions were installed. Institutions as such don't change much even after the end of colonization and persisted over time. Being rich or poor determines the risk of civil conflict (Collier & Hoeffler 2004), and since institutions explain a lot of the income differences, while geography is insignificant in AJR, then the way of how European colonized explains a major part of African instability.
I'm on an old phone and for some reason I'm having trouble editing, terrible typos ahead, I apologize:
This is impossible to quantify, and there's no way to knoe if it would have been more violent had European intervention never happened. What you're essentially asking is "what if Africa had never been colonized, would it have been more or less violent?" As historians, we don't ask "what if" questions.
Also I feel like this question has a certain agenda I would like to address. Of course the answer is that Europe and the Omanis, in the case I the Swahili coast, are indirectly and directly responsible for a mass amount of violence in Africa. It is easy for modern liberals who reject the racist notion that Africans are savage and that explains the violence we hear about. Instead, they say Europe is responsible for much or most I the violence in sub-Saharan Africa. The problem with this notion is that they are still denying Africans some humanity as they portray them as passive subjects to European control of their fate. It also leads to nativism and African claims that it is their cultures, not Europeans cultures, that are civilized and modern, playing into the useless concepts of civilization and modernity for a negative purpose instead of rejecting them.
David Schoenbrun has a great piece on the framing of African History particularly in the Great Lakes region. He argues that the framing of African history with a focus on modernity, and subsequently colonialism as that is usually seen as the start of African modernity (though many pieces try to argue for distinct non-western modernities, though they are still using Eurocentric language in arguing for modernity and, as Dipesh Chakrabarty said, "there is no shame in pre-modernity"), denies Africans the agency to shape their own histories. By framing shifts in African history through Colonialism and only use the paradigm of colonialism to explain Africa today then your starting point for Africam history will always be Europe and the focus will be on Europeans. Schoenbrun himself attempts to explain Great Lakes African history through the framework of public healing, so that colonialism is an important event within the framework instead of the event itself. One can argue about the success he has with his choice of historic lens, but I think his call to not explain African history through colonialism alone is important.
Basically, of course colonialism goes a long way to explaining a diverse range of conflicts in Africa, but you should look for other explanations more rooted in the African and not European context as well.
Chakrabarty quote (need access to American Historical Review): http://m.ahr.oxfordjournals.org/content/116/3/663
Schoenbrun article (need access to JSTOR, though it can probably be found elsewhere): http://www.jstor.org/discover/10.1086/ahr.111.5.1403?searchUri=%2Faction%2FdoBasicSearch%3FQuery%3DDavid%2BSchoenbrun%2Bpublic%2Bhealing%26amp%3Bprq%3DDavid%2BSchoenbrun%26amp%3Bhp%3D25%26amp%3Bacc%3Doff%26amp%3Bwc%3Don%26amp%3Bfc%3Doff%26amp%3Bso%3Drel&resultItemClick=true&Search=yes&searchText=David&searchText=Schoenbrun&searchText=public&searchText=healing&uid=3739688&uid=2460338175&uid=2460337855&uid=2134&uid=2&uid=70&uid=4&uid=83&uid=63&uid=3739256&sid=21104126796391
On a phone, sorry if there are typos and no proper citation.
I would say you could make that argument that practically all of Africa's instability issues have some basis in European Imperialism/The Western Dominated Political System... If you want to read a good book about African Instability I suggest Africa and the International System by Christopher Clapham...
For example you can take the Rwandan Genocide... The denomination of an individual as a Hutu or Tutsi was not a racial difference until the Belgians showed up... Originally it was a distinction between those that worked the land Hutu's and those that owned cattle/land Tutsi... they were fluid terms...A Hutu could purchase cattle and become a Tutsi, or a Tutsi could loose his land/cattle and become a Hutu... when the Belgians arrived they believed these terms were racial and made them concrete through the creation of ID cards labeling you as a Hutu or Tutsi... they also felt that because the Tutsi were landowners they were naturally the more powerful and fit to lead of the two races, and gave them a privileged position over the Hutus and let them run the government, receive education, etc. leading to tensions between the groups that rose and rose and 50 years later you have a genocide...
but even after the end of imperialism, the nature of the international system and western ideals/the political norms of the cold war shaped newly formed African governments to be what they are today (unstable)... Following WWII, state sovereignty became a cornerstone for the international system... (because Hitler violated the sovereignty of countries like Poland and Czech Republic) the international system agreed that you had to respect the borders of international states and let them govern their own people... this conflicts with the cold war mentality of the US and Russia both trying to pull states toward capitalist democracies vs. communist regimes... and Africa became a major site for proxy wars, since leaders of African nations had two options, and could basically go back and forth to entice the superpowers into supporting their regimes....For Example, Angola where the US military fought Cubans in Angola (which to me just sounds hilarious)... But if an African leader felt he was going to loose power he could tell the United States the political opposition group are communists you don't want to see me out of power... and we would support them regardless of what they were actually doing to their people and because of state sovereignty we felt justified in not caring because hey its their country... Thus African states evolved in this model... where they're not accountable to their populations but to the powers in the international system because they DO NOT GENERATE REVENUE FROM TAXING CITIZENS! and can generate a lot more revenue from Foreign Direct Investment... Thus the governments have adapted to not give a shit about their people making them prone to protests and instability
Sorry I went kind of on a tangent their but I hope that helps answer your question a little bit
Thanks for the replies everyone! Imperialism is an interesting subject to me so if any of you have reading recommendations on any form (Japanese, British, Spanish, etc.) please feel free to let me know.
If you are interested in Sudan and South Sudan, I highly recommend Douglas H. Johnson's "The Root Causes of Sudan's Civil Wars." He is considered one of the utmost experts on Sudan's history and goes into great detail about the effect of colonialism on the country.