Flor I don't believe you can put mercy killing and the death penalty in the same boat. Mercy killing is helping someone to commit suicide who is unable to do it themselves do to a medical condition. They may be in extreme pain or vegetables. Either way, they do not wish to live anymore, and have made it clear that THEY want to die. If it wasn't for medical science many would have died long ago of natural causes. For the religious amongst you, you could look at it as giving God a helping hand.

The death penalty kills healthy human beings who have broken the law. Presumably they don't wish to die, and are not currently dying of natural causes. Also, they may have been wrongly found guilty of the crime, or may suffer from mental problems that make it difficult to hold them responsible for their crime (i.e low IQ, insanity, etc). Many come from abusive or impoverished backgrounds, etc. For the religious amongst you, you are playing God, and taking his rightful punishment away from him.