Shocked to see no one's suggested Tyranny yet which is an isometric RPG where you specifically play one of the bad guys. Yeah you can make some "good" decisions but ultimately, you're a foot soldier for the bad guys. It's got plenty of that conflict about what's right and wrong that you're looking for too!

Disco Elysium's protagonist is a walking disaster and there are a lot of ways you can play him. Honestly playing him as a totally morally upright professional is one of the harder ways to play. You definitely don't feel like a hero while you're playing

Not even one mention of Prototype. What have the world become.

Factorio, you literally colonize alien species and pollute their worlds because you feel like it.

The GTA series might be a good example. All of the protagonists of the games commit exceedingly worse crimes as the game progresses but they're made to be sympathetic since they just want success in a world with not much other opportunities.

Games with karma systems may work as well if the bad option isn't overtly evil. I'm thinking games like Dishonored, Fable 3, Undertale, or any Bethesda game.

Anti-hero protagonists like Kratos from God of War, Arthur Morgan from Red Dead 2, and V from Cyberpunk could also somewhat fit the bill.

Edit: just watched an Outside Xbox video that summarizes a few games with just this premise.

Planescape: Torment
Disco Elysium
Prototype
Crackdown

Already mentioned elsewhere but seconding the recommendations: Infamous I and II Bioshock Infinite

Spec Ops: The Line

No one's said Infamous yet. Shocking. Well Infamous. You get to choose if you're going to be good or bad and it's super satisfying either way. Dishonered has that same sort of choice, also amazing whichever way you go.

Dungeon Keeper 2

Spec Ops: The Line?

Dungeon Keeper.

Destroy all humans.

Spec ops the line.

Braid.

Manhunt.

Saints Row games. They're like GTA, but increasingly over the top in their parody. You're the boss of a criminal gang. 2 is still "normal parody" and a bit dated, 3 jumped the shark while doing a kickflip with a jet ski, 4 is even more insane.

Sleeping Dogs you play as a cop, but you can betray the law and side more with the criminals.

Warcraft 3 (old but gold) - the human campaign of the base game gets you from a hopeful young paladin prince into a cold, vengeful psychopath; the following undead campaign is said prince (well, king now) finishing the job of killing everyone and further fucking everything. The expansion has 3 extra campaigns, none with "good guys"

Divinity Original Sin (1 and 2) lets you play as big of an asshole as you'd like. Of the Elder Scrolls games, Morrowind is the one that lets you be the biggest bad guy around (you can still finish the game even if you kill every important npc and break every quest)

Baldur's Gate 3. Not only can you make an evil character, one of the premades is a special evil character called the Dark Urge and basically forces you into doing things that are often even more fucked up than the basic bad guy stuff.

Most cRPGs are like that, actually. Rogue Trader is another good one. Being a paragon of good is nigh impossible in that one, since you're basically a tyrannical land baron in the WH40k universe where everyone is garbage. 😃

Party Hard. You're trying to murder everyone at a series of large parties without getting caught. I guess you're fed up with your neighbour's loud parties. Retro graphics. Humorous. Lots of creative mayhem.

Command & Conquer 3

Carrion

No recommendations, but just wanted to thank you for a real good question.

Well there's always Grand Theft Auto.

This might produce some arguments, but The Last of Us. Joel is far from a good guy. This is established in the first few minutes of the game and you get enough "hints" about that throughout.

There's actually a pretty new video about that on Second Wind: https://youtu.be/fY1FsMK_cos

Other more obvious games are many many Star Wars games where you can either choose or outright start as the bad guy, like TIE Fighter, the Jedi Knight series, Knights of the Old Republic 1+2 + The Old Republic, Squadrons, those Battlefield games and the Age of Empires clones (can't remember the names) and Force Commander as well I think. Oooh and Rebellion.

minecraft

you show up in a mostly peaceful world and just start destroying it, enslaving villagers and treating peaceful mobs inhumanely to farm them so you can go kill more mobs that were leaving you alone

Civilization. Choose Ghandi. You're welcome.

Fallout New Vegas

The number of opportunities the game gives you to be an absolute piece of shit is ridiculous.

Here are some examples:

Get the Terrifying Presence perk and literally become evil incarnate: https://www.youtube.com/watch?v=JXBC2OYLM0k

Or just level up speech and be mean in general: https://www.youtube.com/watch?v=STptNbHOXWg

Black and White

Carrion

It's a modern 2D pixel art reverse horror game, it's real fun.

Overlord, the first game

Prototype

Came back to say Fable and Fable II are peak evil path RPGs.

Well, I may as well keep plugging my current obsession, Warhammer 40k: Rogue Trader. It's a CRPG somewhere between BG3 and XCOM in gameplay, with a grimdark-flavored splash of Mass Effect.

You have 3 "convictions" that many lot of your decisions fall under. Iconoclast is kind of your standard good guy, but maybe somewhat naive trying to be that good guy in the Warhammer verse. But more relevant to you are Dogmatic and Heretic.

Heretic is pretty much evil as far as I can tell. Chaos worship and slaughter for power. Dogmatic is more like Judge Dredd, maybe? You make some harsh fucking decisions as dogmatic, like liquefying a few thousand people to power a computer you need to use, but you are doing it for what your character truly believes is the greater good of the Imperium.

Pathfinder: Wrath of the Righteous, Owlcat's previous game, has some baddie paths too. You're still fighting demons, but it's more of an evil vs evil in some cases. Especially Lich or Swarm.

Fable

Echoing Spec Ops: The Line.

It's no longer available on Steam, but if you can find a way to play it then you should. Probably the most necessary game about war at the moment.

Evil Genius.

Some of the better RPGs allow you to play as an evil guy (equally fleshed out as the good guy playthrough), e.g. Baldur's Gate 3 (it even has a special evil background storyline called The Dark Urge) or (some of?) the Mass Effect titles.

Cult of the Lamb is also a great game where you play a cult leader. Although the game's design is strong on the entertaining/funny side, it's kind of dark also.

Stellaris is a grand strategy game where you can play anything, from a ruthless ruler trying to destroy the whole galaxy to a pacifist trader.

Spec Ops the Line is probably what you want. It's not open world, but the story is excellent.
Bard's Tale is old, but the story is fun and you aren't really a good guy.
Scarface is a GTA reskin that retcons the end of the movie to have Tony Montana live to build a cocaine empire.

KOTOR