Are you fucking kidding me?
You want to omit an array of legal processes and safe gaurds so that the state can simply convict a man (or women) of a crime they deem punishable by death and slaughter him (or her) by means of a bullet in the head?

This begs another question, who is going to pull the trigger?
Im not religious in the least bit, but I still see a major problem with convicting an alleged criminal, then swiftly shooting them in the back of the head.

There are numerous other points one could make, such as that killing a person for a crime does not reverse the damage they have done. It also allows them a quick and easy escape, a physical death. As opposed to having them sit desolate and regretful, confined in a prison cell for the rest of their human lives, a more psychological death.