Why the death penalty is an abomination before God--

I'll start by saying that it doesn't matter one wit whether the person is guilty.  And it doesn't really matter whether you believe the bible or not.  This is one of those things that's a matter of logic--the bible says this, so the death penalty is evil.  You can reach my conclusion even if you think the bible is a fairy tale.  You just need to take it seriously as literature.

I guess a prerequisite for thinking about this is reading the bible like you have a brain.  So, don't just say "well, the bible says 'eye for an eye'".  That was written (according to the Talmud) as a rule to keep people from going too far.  If someone pokes out your eye, you can't kill them.  It was an upper bound.  This is what centuries of serious Jewish thinking has concluded.

So, what does that leave?  Well, you have to interpret the death penalty in the context of the thrust and meaning of the bible.  And the bible, above else, is really the story (ies) of redemption and rescue.  Look at the story Joseph and his brothers. They sold him into slavery.  Their own brother.  (Apparently, he was kind of a prick, but you still can't do that.)  Then they turn up in Egypt starving, and what does Joseph do?  He doesn't say "Oh, well, I'd like to help you out, but you know how an eye for an eye works.  Sorry. Off with their heads"

No, he embraces them as his brothers and forgives them.

When we as a society kill someone, we really are robbing them of a chance at redemption.  Redemption is God's business and we have no right to deny that.  It's really evil at its core.  That's the bottom line for evil. We make our own judgment a substitute for God's order.