My hypothesis is that some people perceive downvotes as rewards (maybe they love to make people angry, or they feel they are on a crusade and the downvotes mean they successfully hurt the enemy)

When I think others are wrong, and in particular, the groupthink is wrong, I take downvotes as a greater indication that someone needs to get their head straight, and it could be them or me. Let's see.

I can think of at least one case where I criticized someone for something I thought was disgraceful, after his post was massively upvoted. I was massively downvoted in turn, but eventually convinced the original poster that they had crossed a line in their original post. Or at least he so indicated. Maybe he was just humoring the crazy person.

maybe they love to make people angry, or they feel they are on a crusade and the downvotes mean they successfully hurt the enemy

Downvotes are a signal. Big downvotes are a big signal.

Maybe it's not about hurting people. Maybe it's about identifying contradiction as the place to look for bad ideas that need fixing.