OK, I'll try and explain it to you.
You're taking a large group of people, lumped together by profession. You are unable to say anything about their actions in a collective sense (there is likely no single action that all of them has performed). You've never met any of them. You haven't even been on the same continent as them. And yet you think YOU have the right to casually say that none of them have a right to life, but should be killed on sight.
It is a brutal, arrogant and despicable attitude to other human beings.
The scenario though DOES involve a specific action (abuse of blacks). As stated above too, you could use the same style of argument to the SS.
The scenario, yes. However, in your initial post you move from the specific scenario to the general statement that police men in RSA could be shot for no specific reason because all of them were scumbags without exceptions, period.
And I don't buy the SS analogy. Their range of activities was, I think, narrower than being a police officer. But, no, I don't think that if someone came across a former member of the SS that mere fact wouldn't be a valid reason for gunning the guy down on sight. I believe people hold responsibilities for specific actions, but not for membership in a group.
And above all, punishements aside, the judging of people who existed in different circumstances in such a sweeping way is something I cannot agree with. You seem to think those people don't deserve to live because they were a part of an evil system. But in a country like Nazi-Germany lots of people were part of an evil system. I don't think all of those people were necessarily inherently evil. I'm not saying that all sorts of despicable actions can be excused. If you were a concentration camp guard you cannot escape that responsibility. But merely being a non-revolter does not merit death, imo. Do you think every white person in the South who lived before 1964 should die too?