Capital punishment is a pretty bad vengeance. Will a serial killer suffer as much as his victims did? No. If you would like a direct physical punishment to get vengeance, molestation is much better. Thats a real punishment. Cut of the criminal's arms, legs and testicles. Stick out his eyes. That's real vengeance. If you want the criminal taken away for good, so he can never hurt other people again, than you can probably kill him. But then you have to have almost 100% evidence, like a surveillance camera actually catching him in the deed. But capital punishment as vengeance only is childish.
In cases of 100% absolute proof, I would support crucifiction or some other slow, painfull, humiliating death. I liked that idea the one Greek dude came up with. A brass elephant that the convict is placed in. A fire is set below, and the sounds of his screams are tuned into music for the audience to listen to.
