Hitler started the Second world war, If he is dead he couldn't control Germany and he won't be able to start the Great second world war right? And if you are a Jew at that time do you want to be kill? Of couse not. So I will kill Hitler so that there will be no Second World war and there will be no more dead. :D
Killing Hitler would not have made WW2 any less inevitable or made the Jews any less vulnerable and there were more competent, rational men who might have led Germany more effectively in his stead. If we're trying to undo holocaust and blitzkrieg, taking out Kaiser Wilhelm II would be more efficient.