I go back and forth on this.
I do not think killing is OK, so just because they killed someone still does not make it right to kill them.
There are HUGE things I battle with all the time (mentally) like what if someone is threatening you, your spouse/child/etc?
I have had hypothetical conversations where I try to say, "Oh, I would just try to get away or perhaps injure the person so I could get away without killing them" and the reply usually is, they could sue you for assault, they could come back and finish the job... "Better to be judged by 12 than carried by 6"
BUT
There are some people that are just evil and will not change. Does it make sense to pay to keep them around for their natural life, housing and feeding them?
I don't think so.
What about the wrongfully accused? Evidence that comes up later and clears someone? Can't un-kill them.
My brain hurts.
One day James went missing (he was hiding under a tarp in the backyard playing hide and seek only we didn't know we were playing) I was on a pure adrenaline rush running up and down our street yelling his name and looking for him in sheer panic. I could tell you I never thought I would be able to kill someone but that day if I had saw someone taking my son I would have ended their life without a second thought.
Kids change everything.