You’re sending them to meet their maker. Nothing you can do is going to be a more just punishment than what He does to them.
Any additional pain you cause is just delaying their final judgement, and is more about you than anyone else. If the death penalty isn’t a deterrent to others, a painful death penalty isn’t going to be either.
Exodus 21:24-25
24 Eye for eye, tooth for tooth, hand for hand, foot for foot, 25 Burning for burning, wound for wound, stripe for stripe.
I’m an Old Testament type. I was raised that way and don’t see myself changing. Society was a lot better off when the death penalty was used and was painful.