Diablo? Gets better as a result of the player's actions? In Diablo 1 you fight your way through the church, catacombs, and burning hells and slay Diablo. Only to then shove the soulstone fragment into your own damn forehead, in the belief that you're will is strong enough to contain an archdemon. Canonically, the player character was the Warrior (assuming you were solo), who's the son of Mad King Leoric. End result, the town of Tristram is saved (for now), hundreds of demons are slain, and you've got a archdemon stuck in your head.
Diablo 2 deals with the fallout of the first game. The warrior who defeated Diablo has lost his battle of wills within days or weeks of shoving the soulstone into his forehead. Diablo is now going around summoning forth demons, and releasing his brothers. The player in this game is one of 5 or so different people who are chasing after the Dark Wanderer (Diablo in human form) and desperately trying to stop the tide of Evil being unleashed in his wake. By the end you have multiple settlements wiped out, still others that suffered serious casualties, and a mountain has been destroyed, as well as the Artifact hidden in that mountain, which you had to destroy to prevent it from being corrupted.
Diablo 3 takes place about 18 to 20 years later. In your wake you leave multiple major cities in ruins, an important border keep wrecked, the High Heavens in tatters, and Diablo has just been released yet again after you had previously sealed the archdemon away.
I refuse to play Diablo Immortal, and have not played Diablo 4. But I very much doubt you leave things better after the adventures.
