You bomb somebody and blow their kids up, and they'll sure as fire fly back and bomb you. Same way with smaller, simpler things. Fights and stuff, for example. Only those who risk going too far can possibly find out how far one can go. It speaks for itself.
Friends are precious.
Dont hold a grudge against them. Forgive your enemies. They are people too. Don't Forgive People Too Much. If a person does something four times in a row, he'll probably do it again.