Death Penalty

I can not endorse the idea that the ends justify the means, especially in this case. Also, I don't see how someone's continued existence in a jail cell makes him a threat to society.

Well, there are different cases.

If they're in solitary confinement, they're a danger to themselves (sometimes), but definitely others. Someone who is such a danger is 100% wasting society's resources by keeping them away from us. These persons are typically not able to be rehabilitated (no honest statistics to back this statement up, it was something I read long ago).

In my opinion, if you kill another human being in cold blood, you have forfeited your right to govern your own life once you're caught.