I’ve never actually met anyone like that.
As for karma, I think it’s more to do with what people will have in their next lives, if you subscribe to those schools of thought and belief. What I have come across is the “Law of Rebound”.
Anyway, anyone going around saying they’re putting curses on people shouldn’t be taken seriously in my opinion. The power of a curse is only as much as you believe it yourself, so there’s no need for anything other than your own confidence and good energy.