You don't waive human rights unless he turned into a different species by what he did. They are rights that you have by virtue of being human. Just human. Not a good human or not even not a completely terrible human. Completely terrible humans are still human and still need protection from the arbitrary power of the state (or the powerful whoever that may be).
There are so many arguments against the death penalty and I think people have heard them so often that it's really pointless listing them again.
For me the crux is that we are who we are just by the luck of the draw. More importantly maybe, a society that sanctions killing is pretty much always not very good to the living either.