Hi u/Toomuchdata00100, apologies for the late reply here. I'm afraid I've been busy with the final phases of my PhD so I didn't (and still don't) have much time to reply but will add my 2 cents!

There's two factors to consider here, the actual, historical South African mercenaries, and their popularisation in media. Let's deal with the former first:

From 1966 to 1989, the South African Border War took place in southern Angola involving South Africa, South-West Africa (today Namibia), Cuba, the USSR, China, and the USA. The link I've posted will provide a decent historical overview of the conflict, but essentially it was a Cold War proxy war in which S. Africa was attempting to a) retain control over South-West Africa (SWA) and b) wanted to prevent communism from gaining a foothold in Africa that close to what was essentially a Western Bloc nation (hence the clandestine support of Apartheid South Africa in the conflict by the US).

There is a lot more to the conflict, but an important thing to note is that, much like with the US and the Vietnam War, conscription was implemented in 1967 in S. Africa for young, White men to enter into the South African Defence Force (SADF). The conflict would span 23 years, and conscription only ended around 1994, and during this time men were still expected to attend "camps" (which were duty camps and rotations in the southern African territory) even after their conscription service time had ended. This meant that regular military service was a very common thing for much of S. Africa during this time and also meant that many men made the military their career of choice. It also meant that soldiers of the SADF become highly proficient in anti-guerrilla warfare, and experts in bush and jungle terrain fighting and strategy.

Conflicts were erupting all over Africa during the period of decolonisation, and the SADF had been involved in several other African countries around this time. Thus, it was not unusual for South African soldiers to already have been seen in other African countries that they weren't really "supposed" to be in.

After the Border War ended and S. Africa transitioned to a fully democratic government, the SADF was amalgamated into a new military, the South African National Defence Force (SANDF). At this point, several things happened: we had a dispersal of experienced military personnel scatter around S. Africa, Africa itself, and even globally as many did not support the new government and/or the new military structure and leadership, whereas some simply wanted to leave the country for other prospects or felt disillusioned with the lack of support once they finished their national service. Some also saw an opportunity to begin private security firms in S. Africa and paramilitary organisations outside of S. Africa with their military experience as a basis.

Some of those men also joined mercenary units around the world or formed their own, local, mercenary units where their knowledge of counter-insurgency and bush warfare would make them indispensable. The most famous (and controversial) of these was Executive Outcomes - I recommend listening to this podcast interview with the founder of EO, Eeben Barlow, to get an idea of the impact EO had in Africa. EO pretty much singlehandedly popularised the usage of S. African mercenaries in Africa and they have a very long and involved history in African warfare during their run. However, many SADF veterans also made a decent living offering their skills to various other militaries around the world, and especially during the US military operations in the Middle-East.

All this to say that S. African mercenaries have made a name for themselves fighting around Africa, for the US and other militaries, and their reputations (good and bad) are often well-deserved.

That's it for the historical context, the next to consider is the popular media context. Both are linked and often feed off one another, but it's worth differentiating here seeing as you brought up films such as Blood Diamond etc.

As mentioned, S. African mercenaries and soldiers were already moving around Africa during the Cold War / Border War period (60's - 80's). During this time, Apartheid was especially prevalent in global media and especially anti-Apartheid sentiment was a regular feature on the radio, in film, and on television. We should also remember that the Vietnam War (1955 - 1975) overlapped with the Border War, and thus anti-war sentiment for both wars mixed with anti-Apartheid popular and political sentiment and this bled into popular media at the time.

Lethal Weapon 2 (1989), Splitting Image's I've Never Met a Nice South African (1989) and Midnight Oil's Beds are Burning are three of my favourite examples of this sentiment (though it should be mentioned that Beds are Burning was actually about Aboriginal Rights in Australia, but it was adopted by S. African underground protests as the issues were relatable in SA).

Apartheid activities and the SADF's actions in southern Africa made S. Africa an easy bad guy to popularise, along with their current and former soldiers now dispersing onto the global stage - much like how Soviet Russia was and is often portrayed in a similar bad guy-esque manner.

And much like we still have the stereotype of "evil Russian bad guy", so too do we still have S. African mercenaries portrayed in a similar manner to what they were when this started. And, much like Soviet veterans still involved in certain affairs, so are many S. African mercenaries still very much active and their exploits still sometimes make the news.

Often, mercenary groups are portrayed in an unsanitary light as either they engage in unsanitary work that standard militaries need done but won't risk being associated with, or they take part in shady deals (such as smuggling or coups) for payment - or perhaps both! There are some companies out there that have helped people and are more complicated than we give them credit for, but more often than not, I would argue, they are the work horse of the darker side of military activities.

Anyway, I hope that helps shed some light on your question and feel free to let e know if you have any others, though might take me a while to get back to you :p