If its someone who just went from idealization to extreme devaluation through an act of betrayal, its bad for me. I want to make them feel as much pain as I do. Mostly I just fantasize about it though, now, thankfully. I have hurt people I love(d) really badly in the past with words and actions though.
But yea, lashing out happens...
I am constantly building a defense around people in general - especially the ones who I do not have an idealization of or maybe even just don't like, though, and in that build up I gather information and store hateful remarks in ready so that I will go down kicking back or maybe even appear to be far tougher than I really am - because I expect to be attacked eventually. I keep the remarks in now though through some self-taught methods I am still working on, but sometimes they are just waiting to come out especially when some person I could care less about has tried to hurt or has hurt me. In my mind sometimes I am just sitting there thinking the person has no idea how bad I could hurt them, as I let them win. Letting them do their worst and not responding the way I want to takes almost everything I have sometimes, but it means a lot to me. Self-control.
I take little punches though. I guess you could say when I fight I pull my punches now, just to let them know to back off. Still though, sometimes you can misjudge how much even a small remark will hurt someone. I hurt a friend recently like that, who I felt had betrayed me. It could have been so much worse, but as it was it was bad enough for him to run away for a long time.