I am opposed.
I strongly believe that every person has some good in them. Due to said person's experiences in life, they may have taken quite some wrong decisions, and therefore landed in criminality. Sometimes there just doesn't seem to be a way out.
That and the justice systems seems to be mainly aimed at punishing people, rather than actually trying to help them find the right path in life. I believe more effort should be put into researching how criminals can be helped to see why the laws are in place, and how it does benefit them to abide them.
In the more extreme cases (murderers etc?), I think we'd have to dive into a more psychological level, find out what drove the person to commit these acts, and find remedies etcetera. Show other ways than murder to get the feeling of power/control it may have created in the murderer.
I cannot really tell what exactly causes people to do such monstrous acts, but I strongly believe that everyone can be helped, with the right techniques. This doesn't mean brainwashing people, it means helping them get to insights that were out of reach before.
Now from another point of view, the relatives of a person that was murdered probably simply want revenge for their loss, which is why they might be opposed to not killing the murdered. In fact, I find death penalty quite a lenient punishment, seeing as the criminal is then immediately set free. (However, one might be of the belief that there's nothing after death-which doesn't take away the fact that no suffering is needed, so it still stands). What I was saying, lifelong locking away is a far better punishment then, if you're looking for revenge.
But yeah, to sum it up: From my point of view, everyone is able to get on the right track, with the right help, so I am fully opposed to the death penalty. From a revenge point of view, I am opposed as well, as it sets the criminal free anyway.
Thanks for the interesting topic, SBB.