What would you say to someone who wishes a Holocaust would happen to his bullies and critics? (Can I get some thoughts from a Holocaust survivor or a descendant of one?)

I understand where they are coming from, but I believe that's a bit too much. Actually it's a lot too much. But everybody has their own opinions and we should respect them whether they are right or wrong.

That's a very childish way to feel.  Wishing such a horrible thing to happen to anyone shows someone is ill equipped to deal with life's problems.  Much healthier to learn proactive ways to handle difficult situations instead of having tantrums.

I would let the person know that I hear what they are saying and that I understand why they are feeling that way.

(If you don't acknowledge what is at the moment the most important thing significantly affecting him in his life right now, he's not going to be ready to listen to your "solutions" to "fix" his problem.)

Deal with the feelings first and then go for the "problem solving" phase.