You picked the easiest possible case and demanded people defend the killer.
Most would concede Horner “deserves” to die. But that isn't the question. The question is whether the state should be doing it.
Over 200 people sentenced to death in this country have later been exonerated. Cameron Todd Willingham was almost certainly innocent and Texas executed him anyway.
You can't build a system that only applies to the obvious cases. You get the whole system or none of it, and the whole system kills innocent people. That’s been proven, and there is no greater injustice.
One last thing, since you're Catholic. Our own Church teaches the death penalty is inadmissible. Pope Francis put it in the Catechism. If you want to claim moral authority here, you're offside with Rome.
We don't have to argue Horner deserves to live. We have to argue the state shouldn't be trusted to decide who dies. You of all people have faith in the state, here? No wonder you skipped that argument entirely.