Zack Fair has a really fascinating relationship with killing where a lot of the rest of the cast doesn't. He fights people as well as monsters and gradually stops being able to differentiate the two. He helps clear out beasties and ghoulies but he also intros the game with a massive attack on a foreign nation just to shore up corporate interests and for a good chunk of the game places those two activities in the same spot in his mind. One of his side projects at work is quashing the last remnants of rebellion in said occupied country and rooting out the spies in their resistance, at which point he hands them over to his bosses to be interrogated and presumably tortured. But if he comes face to face with the same people in a combat scenario, he won't kill them, because he views their desire to see themselves free as an honorable trait. Then he turns around and attacks hordes of people who defected Shinra. He uses the blunt side of his sword, not to spare lives, but because he doesn't want to damage the sharp edge.
Wutai forces and Genesis clones have a different categorization to him, something he can mentally label as "other". But after Nibelhiem, your primary enemy type becomes other Shinra soldiers. Just after Zack has had his realizations that the monsters he was wiping out share a haunting amount of basic building blocks with his coworkers. And with himself.
Actually, let's pause. Zack is the only character we see have any sort of acceptance of being Jenova-ed. Every other character is unwilling and usually not even a conscious being yet, but Zack A) knows what is being done to him, and B) openly states he wants those monstrous traits for himself. "Those wings / I want them too." Up to this point, every other character has equated being Jenova-ed to being baser, to being subhuman, but Zack at the very beginning tried to convince Angeal that it meant freedom. Power to do what you want.
Zack Fair willingly and with open arms embraced being made less than human because he thought it would give him the strength to break out of captivity.
So he busts out, and the first thing he's greeted with it a horde of enemies that he knows are human, has worked and talked with, and are in fact the same shape and type of person as Cloud. Just a couple cutscenes ago, these were his coworkers.
The easiest way to get through them is to set them on fire and blow up the munitions they're carrying on their person.
(It's such a beautiful demystifying of the elemental system. I love it.)
Oh also, you're on a time limit, so you gotta kill all these guys fast, because they're trying to go for the weak spot, Cloud, who can't fight and is getting dragged away.
After that, the next thing Zack does is find a gun and start taking long ranged killshots.
There's a tangible feeling of a lesson being learned.
Zack is (I think?) the only SOLDIER we ever see use a gun. It's worth noting that even the most fallen of SOLDIERs stick to their flashy blades and their weird swordfights. Other characters even comment how bizarre it is, but ffvii also runs on that fantasy world logic where if you just Get Good enough, swords are just as good as guns if not better. Guns are the lower, more primitive weapon, fit for grunts and hitmen but not elite fighters.
(I could also go into detail on how some of the SOLDIERS weapon are sold as bespoke merch in world, and basically another tool in the toolbox of how to sell these flesh and blood men as mythologized products. But we don't have time to unpack all of that.)
For Zack, we know that his sword is even a symbol of honorable combat and a legacy of trying to do the right thing. Zack picking up a gun is basically debasing himself to being just another man in the field. And it's a sniper rifle. The weapon guaranteed to get him as little contact with his opponent as possible and does not even give them a chance to fight back. But it's whatever gets the job done.
Zack starts the game as someone who kills people, but it's hand-waved aside the same way it is for most everyone else, only for him to bite that awareness apple like five minutes before it becomes absolutely vital he survive at all costs. Zack becomes aware that he's on a slippery slope and due to circumstances has to start sprinting. Over the course of the game, we watch as Zack Fair goes from being someone who kills people to a killer.