Killing foreigners outside of the own country has always been deemed acceptable by governments that are (or were until recently) considered to generally follow rule of law as well as the majority of their citizen. It also doesn't necessarily contradicts rule of law.

It's just that the West has avoided to do that to each other because they were all essentially allied until recently and because the political implications were deemed too severe.

I don't think however France has anything to win by doing it or has any interest whatsoever and I doubt there's a legal framework the French government can or want to exploit to conduct something like that legally (like calling something an emergency situation or a terrorist group, for example).