EnPenSive yEaH!!'s feed

I'm always wondering(not because it always happen in some manhwa's but also because this thing sometimes happens irl) why do some people who have mental illness or a trauma,they'll do the same to other people for them to also experience what they went through,do they feel fine after ruining another person's life? I'm always trying to find ways to understand on why some of them do this but-