While I agree to some extent, realistically, if we follow that logic, wouldn't we have to feel a sense of melancholy every time we kill an enemy in almost any game?

It's highly debatable whether players actually feel melancholy when fighting monsters in Dark Souls. Putting aside the fact that the story is notoriously cryptic and reliant on player speculation, yes, the lore of Dark Souls is tragic. However, this is the sadness of the 'lore,' not an emotion driven by the 'gameplay.' The problem is that this tragedy must be pieced together by reading flavor text. Does the game actually communicate this naturally during play? Not really. The player is simply thrown into a brutally hostile world and left to suffer. In reality, players hunt these monsters to buy gear or level up, not out of melancholy.

Shadow of the Colossus portrays tragedy brilliantly in this regard because you actively track down and stab peacefully existing creatures. But I strongly question whether Dark Souls and Spec Ops: The Line belong in that same category. Spec Ops: The Line forces you to commit atrocities just to emphasize a protagonist going mad, and in Dark Souls, every monster is inherently hostile toward you. I find it hard to believe a player would feel genuine melancholy from this kind of deceptive design, where the game fixes your choices entirely on a linear track just to force a tragic point.

Normally, when an enemy is that hostile, your only thought is, 'I just need to kill this bastard.' The sadness in Dark Souls is a retroactive feeling you get from piecing together flavor text. While I appreciate the depth of that narrative, it's very hard to put it on the same level as making unprovoked attacks on peaceful monsters (Shadow of the Colossus) or actually having the mechanical choice to spare them (Undertale).

The praise heaped on Spec Ops makes me embarrassed for games as a medium. This is a game that forces you to commit mass murder in order to progress the storyline, pretends that something profound has just been demonstrated, and then tries to guilt trip you about it for the rest of the experience. It's a mess of dumb, trite, "war is hell" cliché and I wish we would collectively forget about it.