Friends
They're called Daleks, and in my world, they're pretty much the be all end all of terrifying races. No emotions except hatred. They're made to destroy. When humanity is inflicted upon them...well, let's just say, it doesn't go well.
See, this is the problem for me. You also run into a time traveler question. Fixed point in time, you can't stop what has to be from being. But if you wanted to make it a proper morality question, I suppose you could ask if the Nazi party should be killed for their crimes. And I think my answer to that would be that their leaders probably should be, yes.