The death penalty has been shown numerous times to have been handed down against innocent people later exonerated by DNA evidence. Also, the death penalty is totally about vengeance and not deterrence.
The death penalty goes hand-in-hand with anti-rehabilitation. The people who support the death penalty and locking them up and throwing away the key are hardhearted. It is that very hardheartedness that is contributing so much to most of the crimes in the first place.
Human vengeance is not justice. That's why God says to leave vengeance to him. He is just. No innocent person will be punished in the afterlife by God. In fact, it is Satan who does the punishing in Hell.
Lastly, there are many who like the whole prison system because it is ever more the profit center now. It used to be used for convict labor, but now, the prisons are being corporatized and privatized. What stockholder in a profit-making prison corporation is going to vote against more incarcerations on moral grounds? The vast, vast majority of those stockholders will not vote against their stock values. It's a nasty business that no real Christian can ever support.