…would we still do it?
Personally I can only see it being used for life-or-death circumstances. We wouldn’t go around killing people just because it’s socially acceptable. Besides, if people just kept killing each other for the most trivial of reasons there’d be nobody left!
Would it be socially acceptable to kill a pet, a child or somebody who is pregnant? I don’t think so. Taboos against harming pets, children and the pregnant are ingrained in most societies. It would take a MASSIVE extenuating circumstance to do so in my opinion.
Would murder motivated by a factor of identity- for example, race or sexuality, be accepted? Definitely not. And identity-motivated killings cannot be successfully excused away as “self-defence” or “my finger slipped while I was calling sem some racial slurs”.
Of course this is an unlikely scenario. Most modern religions prohibit murder and that has become so ingrained even in the most secular of societies. This won’t change for a good long time, perhaps even forever.