No one group was particularly responsible for the casualties that occurred. One of the reasons why African civil wars in particular see an especially high civilian death v military death ratio is the incredible fragility of their economies and infrastructure. In much of Sub-Saharan Africa, especially in the pre-cellphone era (I cannot overstate how much cellphones have changed the continent) you basically had pre-medieval farming practices, a more modern sized population and a light amount of modern infrastructure bolted onto the whole system trying to keep it going. To make it worse, colonial administrations wanted to ensure that they could sell goods from the motherland in each of their colonies, resulting in them wiping out a good deal of local consumer goods and tools production. This results in an incredible fragility with a relative lack of self sufficiency in each local areas.

Unlike medieval farming villages, which had a high degree of in house production, much of Sub-Saharan Africa imports most of its consumer goods, agricultural tools and support and medicine. This is not a terrible thing in and of itself. Thanks to the large amount of hard capital goods (cash crops, minerals, ect), many of these countries are able to maintain a positive or neutral balance of trade as long as the system remains functioning and resources get a good price on international markets. However, civil wars have a nasty habit of completely collapsing internal systems. More importantly, much of rural Sub-Saharan Africa has very little infrastructure, for example, in Angola itself much of the highlands were connected to the capital by a single road. This means that if that road gets cut off, which would be crucial for any insurgency or rebel force to do if they wanted to prevent enemy forces in the highlands from being supplied, this also cuts off much of the civilian support network.

This was the same thing that happened in the Dem. Rep of the Congo. Out of the close to 5 million people that died during the two Great African Wars, many of them died not from direct causes but indirect causes. While violence or deliberate starvation claimed an immense amount of lives, most of those who died were killed by diseases that otherwise would have been treatable in more peaceful times, or starved because of the collapse in internal trade.

To add on to all of this, many of these civil wars also raise armed banditry on a level which is very rarely seen. The proliferation of weapons and the lack of organization on the part of most sides in these conflicts result in "armed units' that are basically there to loot the place with the occasional clash with opposing forces. This impacts the hard goods mentioned above, alongside the need for increased military expenditure, further reducing the ability to acquire and move the crucial imported goods that keep the regions alive.

This also contributes to the long duration of many of these civil wars. The large amount of indirect casualties (starvation, disease) in a way keeps civilian war weariness down a bit as you aren't directly linking armed men to much of the suffering going on. More importantly, even if such weariness were to emerge, the relatively small number of fighting men vis a vis the population and the fact that most arms are imported means that what the population wants is relatively unimportant. Seizing food from the population is pretty much all the interaction that is necessary to keep a conflict going, as well as conscripting laborers and taxing natural resources. As such, the sort of total civilian collapse as seen in Germany during the First World War, doesn't really occur, allowing groups to keep on fighting year after year.

In summary, no one group is especially to blame. Unlike many Western Conflicts, where civilian casualties primarily come from the direct actions or consequences of military actions, in many Sub-Saharan African civil wars, including the Angolan civil war, most civilian deaths are a product of the incredible fragility of internal economies, and food and medical supply networks. In short, the war itself and the environment it produces is the true killer.

Sources: Maier, Angola, Promises and Lies Steams, Dancing in the Glory of Monsters, Emerson, The Battle for Mozambique