I don't believe in punishment at all - I believe in treatment.
If a person cannot be treated, then I believe in killing them as fast and cheap as possible. I'd opt for deportation, but the globe has run out of unclaimed territory. One might say noone has to the right to take life. I say noone has the right to say who has the right. I say there is no right.
I do realize that punishment means that the masses' hunger for revenge gets satisfied. And I also realize that punishment is an effective means of terror. I don't put much stock in either, to be honest - on the contrary. One is an indication of doubt and hate, directed at a demonized individual criminal.
The other has been proven time and again to be misguided and outright false. The US has some of the harshest punishments in the world - it's had that as long as it's existed. It's also had one of the highest crime rates. The idea that harsh punishments mean less crime is simply ignorance.
And really, if there should be capital punishment for something, it should be ignorance... And lying. =)