Still not a reason to kill someone...at least not in my opinion.
A few people have already said it, but I think the best phrase I've seen yet is "An eye for an eye makes the whole world blind". There is no logic behind "hey, you killed someone so now we will kill you!". The death penalty is one of the most useless and hypocritical punishments ever conceived.
I agree that there should be boundaries and starting by actually implementing life sentences would be a great start.
I can't speak for other people, only for how I view it. We have a rapist/murderer called Marc Dutroux.
He kidnapped, raped (multiple times) 6 girls, kept them in his basement for years and later buried them alive.
In 2004 he got life and now 10 years later he's asking for an ankle monitor so he can be released from his cell.
And in my opinion I just don't believe he should ever get free, let alone waste our tax money on him because he has the most luxurious cell in the country. (TV, cell for him alone, he eats better than most of the poor people here.)
His wife (accomplice) got the same verdict and was released earlier this year, and I'm sure, if it wasn't from people protecting her she'd be dead a countless amount of times now.