Islam doesn’t support lay interpretation of doctrine. It would have to be sanctioned by an Imam.
What you are probably not quite figuring out is that there are Islamic countries (57 of them) that have, at least in part, a modern civil and criminal code.
Each of those countries continues to have large pluralities of folks who like to commit mayhem on their own understanding of "kill". Doesn't mean they won't get punished by the government, but that's a whole different matter.