Star Wars: Jedi Academy. Not for much of anything that anyone responds to in the story, the betrayal/dark side bit was visible roughly from the moment I met the character, but for how much killing you have to do added to the right mental conditioning.
Playing several years ago, I didn't so much as give a sideways glance to running countless warm bodies through with a much warmer lightsaber, frying tusken raiders with force lightning while invading their territory, throwing petty crooks were probably just desperate to make a living from the tops of buildings so high the floor was an invisible black mass, and expending any leftover explosive rounds on harmless jawas. All in good fun.
Between then and the next time I played, I'd become something of a compulsive role-player, trying to act in ways that make more sense than "Hello simulations, I'm here to pretend-murder you." So getting into the mindset I used in KOTOR, where pacifistic solutions were a thing, even if not a constant, I couldn't help notice that my light-side jedi character (forced name Jaden Korr) was more concerned with making quips and sarcasm than snuffing out 67 or so lives per mission. The game helpfully keeps count.
So, we have the "good guys" killing tons and tons of people because said people are usually up to something that may result in even more senseless death, like releasing a super rancor (that brown, snarly thing in the pit in Return of the Jedi on space steroids) into a very populated city. I'm reminded of Luke killing everyone on the Death Star (both of them, really) because it had a laser that blew up entire planets.
So yes, bleak, but the lightsaber fighting isn't any less fun for it all!