The Death Penalty - Total Abolition?

I have often wondered why leftists are always so anxious to see the total abolition of the death penalty. It makes no sense at all.
If you want it restricted - rarely if ever used - I can perhaps understand that.
But just how do you predetermine that nobody in the future could ever do anything that might warrant the ultimate penalty?
Furthermore, even the faintest possibility of a deterrent effect is lost.
There is a presumption from these undemocratic people that they hold the moral high ground when all they are doing is granting freedom to all would-be murderers to kill in the full knowledge that they are able to do so without their life ever coming under the slightest risk.
Moral high ground? - Hardly!