I'd argue that because some people deserve to die, that doesn't mean that you should kill them. I firmly believe that the only "good" motive for killing someone should be to prevent greater suffering and death.
If you just kill criminals, even those who have done terrible things, you lose a little of your own humanity - become a little more like the thing you're trying to stop. A society that kills is not in nearly such a good moral position to condemn killing as one that sees the inhumane treatment criminals and killers mete out and then refuses to sink to that level in response. I'd rather see murderers in prison than dead, because death is an escape from the reality of what they have done.
Secondly, I think it's dangerous to give the state a statute book with the power to execute citizens. If it's not on statute and there are bills preventing it, it then makes it vastly more difficult for any future leader to use such powers wrongly. I know we may trust our governments now, but one of the things about human rights is that you guard them against an uncertain world not a certain one.
