But you’ll always risk that there’s some psycho who likes killing and then you will have killed more.

I disagree. The blood is not on your hands.

Suppose you see someone walking towards a bank with a gun. You have an opportunity to steal their gun. If you don’t, and they go on to kill 5 people in an armed robbery, is the blood on your hands?

Suppose you see a hunter in the woods with a gun. You have an opportunity to kill them. If you don’t, and they go fire on a city street and kill 5 people, is the blood on your hands?

Suppose you see a juvenile delinquent on the path to being a serial killer. You have an opportunity to kill an old lady in front of them to scare them straight. If you don’t, and they go on to kill 5 people, is the blood on your hands?

Suppose you see a newborn baby. You have an opportunity to kill them. If you don’t, and they grow up to become a terrorist and kill 5 people, is the blood on your hands?