FATE: Murderer-Aspect
There are a lot of good answers already but I actually encountered this exact situation when being DM, so here is how I did.
I was playing a sci-fi FATE game with a group of players most used to D&D. It was clear that they thought that killing the opposition outright was standard procedure which really did not match this more realistic setting.
What I did was that I declared that killing someone in cold blood is akin to an Extreme ConsequeceConsequence:
a very serious cost—you must replace one of your aspects (except the high concept, that’s off limits) with the extreme consequence. That’s right, an extreme consequence is so serious that taking it literally changes who you are.
That is, if they kill someone, they need to replace one of their aspects with "Murderer" or something like it. (I did not count it against the limit of one extreme consequence though).
This worked well to emphasize the severity of the act and also brought with it several other interesting side-effects:
'Murderers' can tag their aspect when they attack or kill people - making jaded murderers more effective combatants and more scary enemies.
NPC's with the Murderer aspect became extra scary.
While one might think that the above is an advantage, it did more to emphasize that a murderer is a horrible thing to be avoided rather than looking like a mechanical advantage.
The murderer aspect can be compelled, tagged and invoked in other situations - anything between attacks of bad conscience, problems with religious faith, problems with law enforcement etc.
As a result all players except one steered clear of killing and a D&D mentality. The one player who did not got seen by the others as a scary psychopath which also led to interesting roleplaying.
I will definitely do the same if I run another realistic FATE game (or system where such a thing works mechanically).