Malema has not called for the killing of white people - at this time.
Once the actual genocide begins, it will be too late to do anything about it. Attacking Trump because he took notice of the risk before this racial cleansing actually began will only heighten his approval rate
if when it takes place. Another reason why the left is losing.
What will happen in S. Africa exactly is a good question. Malema may have been completely serious about not killing white people; he's trying to walk a fine line in keeping from being economically isolated at best and invaded at worst. It's possible that most white farmers will simply be disposessed and moved to protective custody in
concentration refugee camps, where they'll likely be ignored to death.
Would they be rescued by the US? Not impossible, depending on who's in the driver's seat. Would other countries rattle sabers at such an invasion? Russia might make muted noises; they've already come down on the side of the Boers. China would view this with alarm and step up their rhetoric.
What will come next in Africa? IMO China, they've been building some immense cities there and are moving people in. Once economies collapse (further) the African population will covet what the Chinese have built and attempt to retake that land as well. I'd expect the Chinese to respond with more than small arms, and possibly use the excuse to conquer larger swaths of territory. How the West responds to that will be fun to watch.