It seems that people will go on killing people regardless of culture and political states--be it through war, blood lust, vengeance, or social justice.
To me, this suggests that 1) the death penalty is ineffective as a deterrent and 2) it will continue to exist as long as societies do.
I personally don't like the idea; I think it is hypocritical to assign something as "bad" and punish it in the same manner (killing in individual self-defense is different in my view, however), especially when it is done in vengeance and justice. Many a killer on death row, I am sure, felt they were killing for the same reason.
That said, I can still imagine cases where I would support capital punishment.
Still, my first statement stands firm. We like to kill each other. (Or, perhaps more appropriately, we can't help it.)