Since Mandela died I've been occasionally looking up South African stuff from the 90's. I was born in 1992 so I don't have any memory of what was going on, but the videos are very confusing. The Kill The Boer song I know springs from colonialism, but its current use seems so contradictory. I see South African Flags, Communist Flags, people calling for white genocide wearing Mandela's face on a shirt, White people marching with black people while they all sing the song, people with large amounts of guns, and from the documentary I watched on PBS Mandela was arming people from the beginning for a violent revolution. Yet... the world heralds him as a man of peace and equality? I'm really very confused. Can someone explain the whole picture to me? I'm sure there is something getting lost between cultures, but I'm not sure what.
Regarding the specific question about Mandela, I'll refer you to this section of the FAQ. It has some great answers about Mandela's life and the seeming contradictions with how the West portrays him. You can also read up in the Mandela thread as well as the southern Africa thread.
While I'm sure others will give you detailed answers about this specific case, I'd like to talk a bit about songs and slogans like "Kill The Boer". These types of songs/slogans are very commonplace and almost always cause quite a bit of controversy. Other examples include "Eat The Rich", "A.C.A.B. (All Cops Are Bastards)", "Fuck the Police", post-revolutionary use of the sans-culottes version of "ça ira", "Die Cis Scum", and so on. As you can see, these all have similar elements. They're usually used by the oppressed (or those who identify as such), they're very aggressive, not very nuanced and are intended to shock. And they do shock. However, another element they all have in common is that they're not to be taken literally. The song "Kill The Boer" is as much a call to kill all the Boer as "Fuck the Police" is to have sex with cops or as "Eat The Rich" is promoting cannibalism. These songs and slogans are more about replying against very real oppression and aggression with the few methods the oppressed have left: with a song or a slogan.
And while they're aimed towards their oppressors, quite aggressively so, these oppressors aren't the intended target. Not in the least because the slogans and songs will never do much more than ruffle a few feathers. The real goal of these expressions is a bit more vague - it channels aggression and frustration, serves as an act of defiance, enhances cohesion and solidarity in the oppressed group. A good comparison, I find, is the Palestinian child throwing a rock at an Israeli tank. We've all seen the image, but it's hard to understand the logic there. The rock won't even scuff the tank's paint, it looks like a completely useless - and even unnecessarily dangerous - act. It seems unreasonable. And it is. It's a very emotional reaction to powerlessness, to seeing your oppressor and knowing you can do nothing to stop them. You lash out. But does that mean that you're now a valid target for that tank? No, of course not. That would be ridiculous. And the same is true for songs like "Kill the Boer", even though the South African Supreme Court disagrees with me on that one. (It's officially hate speech, IIRC)
But y'know, let that be a bit of theoretical context, I guess. Be sure to check out those links, anyway. There's a great deal of information in there.