Just like everybody else, I react emotionally to some news stories and not to others (often more important). I find the murder of Zimbabweans and Somalis especially sickening (somehow I think it's worth mentioning that there are places where life is worse than Zimbabwe).
To me the act seems to say, "You are not entitled to anything. Even the attempt to survive is a capital crime." When I hear about other murders, I subconsciously imagine some underlying "reason". "That guy is part of a rival gang", "that guy slept with my girlfriend"; "I'm a fucking psycho who likes to look of blood by moonlight" or whatever. It's never OK, but I doubt there are many people who kill for the pleasure of it; those who don't feel immediate remorse usually either think the victim had it coming or they want to get ahead, politically or materially or whatever.
The simplified story in the current violence has involved foreigners taking jobs that South Africans would otherwise get. So I guess both of these reasons apply, but I can't think of it like this. I see it as a punishment for the crime of trying to escape being murdered or facing a high probability of terrible suffering and premature death. Like an ordinary Austrian finding a Jew who'd escaped from Auschwitz and instead of sending of sending him back (because that is the right fate for a person who happens to be born Jewish), executing him on the spot, not for being Jewish, but for the crime of escaping, believing he has a right, not to have his life protected by anyone else, but the right to fight for himself.