It had to happen eventually.
Not all the timelines Gaster entered could be happy, cheery, and bursting with people. He had seen timelines where the Underground had been laid to waste, where everything was coated in a layer of dust, and that was upsetting enough, but this was far worse.
He could hear the screams of dying monsters echoing through the tunnels of Waterfall. Behind him hid an alcove of denizens. This wasn't his home, but he had made them a promise to find a safe way out, somewhere in Hotland or Snowdin where the danger might pass. The human never backtracked, right? He would find the way. He would.
He did. Water soaked his clothes as he ran, an unseen shadow in dark clothes passing through dark tunnels. He had heard Undyne's shrieks of wrath, her final, fading cries. He went unseen.
He was too late. The alcove where he had promised certain safety was empty. The walls were smeared with dust; a broken bottle, a weak defense, bobbed in the water. Inaction or intervention, it didn't matter. He had tried, and he couldn't save them. He knew genocides happened. Why did he think his presence would have any impact? Why did he think he could stop this? When would he learn that he couldn't save anyone? He could never save anyone. He was always too late.
Cold water gathered at the corners of his sockets, and he shivered as he felt his tears pour down both the inside and outside of his face. He could never save anyone. Sinking to his knees, he keened, and he cried, cried, cried.