Killing in the name of God is wrong by default, of we consider that God loves each human. But if God loves every single person, God still doesn’t interfere in this bloodshed.
Potential punishments? Make them clean the ‘mess’ that they created(bodies, blood, etc.), pay for damages in money/equipment, and potentially life sentences.
I know, sound very basic, but that can realistically work.
Death is always an option. No matter what. It’s a valid option, like it or not.