>>542449141
Horror can be about many things and done in many different ways with more or less focus, constant thrills or slow buildups to the few scares it may hold in order to make them more effective. There's no objective way to define what is and isn't horror, you need only look at the vast history of both horror films and horror games to know that it can be rather arbitrary in a lot of cases.
I've personally never played Half-Life before although they've been in my backlog for a long time now and while I know the "horror" tag on Steam is also used for games that just contain "horror elements" every single HL title including the newly released RTX remaster of HL2 has the horror tag except HL1 and Alyx has it as one of the top tags. I think I read somewhere that Valve did consider at least one of them a horror game though but don't quote me on that it might've been BS.

As far as dystopian sci-fi games go I would consider the first two Bioshock games as horror and the System Shock games personally but they do actually have horror as a core focus which I don't know if that's the case for Half-Life.

>>542450287
Left 4 Dead is a horror game because it was envisioned as one originally and Gaben allegedly thought zombies were too cheesy for the project at the time: https://kotaku.com/left-4-dead-l4d-zombies-valve-gabe-newell-chet-faliszek-1848900909
Valve even outright advertises both L4D1 and L4D2 as coop action horror games on their Steam Store pages.
>Horror needs a focus, rather than it being an excuse to have monsters in the setting.
L4D, Killing Floor, Dead Island all fit this bill yet its creators consider them horror games. I don't think it's as simple as you're making it out.