Why do some people believe we have to wait for justice to be served in the afterlife?

Especially if someone does something really gruesome (like Hitler, Ted Bundy, Jeffrey Dahmer, etc.) some people like to use the excuse that the death penalty is wrong and that they will receive punishment in the afterlife. But why should we wait for the afterlife to give them punishment? What if their punishment is not the kind of punishment you think they deserve? Some people try to say that God would punish them, but who is to say that God would give them a just punishment?