I agree with the logic presented here, in that killing a even violent criminal creates some serious bad karma for the killer, and reduces the potential for a peaceful death for the criminal.

But it should be pointed out that it would be very unlikely for the criminal to be reborn human. Rebirth as a human is a very special thing, it is by no means guaranteed, or even likely (especially for a violent person). I read in "The Tibetan Book of Living and Dying", by Sogyal Rinpoche that:

Tibetans imagine a single turtle swimming in the ocean. And a single wooden hoop floating on the surface of the ocean. The turtle surfaces every thousand years or so. Tibetans liken the frequency of a human rebirth to the turtle surfacing with its head in the hoop.
So while I don't dispute that there is a karmic connection between the death penalty and other kinds of violence within society, I don't think it is quite as simple as the writeup makes it seem.

I'm also interested in the other writeup, by DMan. Did the Dalai Lama really have a torture chamber in his palace? It seems totally out of character at any rate. How can I find out more?