Same. I remember getting bullied and beaten up throughout my early years, but I would never put someone else through that because I remember how it made me feel and how much it sucked.
As for why some people grow up to punish the underdogs as you put it? I’ve always wondered that myself. That’s the biggest reason why I think I am so interested in serial killers-that psychological aspect. As you said, there are SO many people who were abused or tortured or neglected or went through horrific things themselves as kids, but they don’t grow up to be killers. But some do. And I think there are a number of factors that contribute to it: neglect, parental/family involvement in their children’s lives, trauma, sometimes brain damage could be a factor, etc.
Sometimes I also think there are a few rare cases where someone was just born evil (I know this is probably a controversial statement) and the darkness that lives inside them just lays dormant until their later years, when certain factors or events come together and create a perfect storm. They could have a perfectly happy and “normal” childhood, with loving parental figures, but still grow up to be killers.