The Chapter 14 with Bill's Round Game. In this game, Dipper meets a guy he knew and is unable to save him from the other demons.
The finale. Bill dies, killed by another stronger demon. That's just...heartwrenching when it's actually happening.
Especially because Dipper, Bill and Ford are all busy fighting the demons. When Dipper realizes what's happening, he runs toward him, desperately trying to reach him and save him. Dipper kills the demon but Bill still dies.
Overall, the context of the last chapter. Throughout the whole story, readers were expected to see Bill be killed by Dipper. Even though Dipper loved him, he still was ready to kill him if he decided to destroy the human race. Even Filly warned Dipper that killing Bill would hurt him. At least, Dipper didn't have to do this.
Bill's last words to Dipper.
"Bill: Pine Tree?
"Dipper looked Bill in the eyes and tried his best to blink away the tears making his vision blurry. “Yeah?”
"The once vibrant, golden eyes were now dulled, and looked more human than Dipper ever thought they could. “I’m sorry.” Bill said it so quietly that Dipper almost didn’t pick it up. Bill’s hand slipped from his face, and Dipper felt a weird shudder go through his body.
The last lines of the chapter where Dipper is in Bill's room, mourning.
The second to last chapter. You can read how Bill and Dipper's relationship is developping and just how Dipper and Ford's relationship is degrading.
Though Doubled as a Funny Moment, there is Bill proposing to Dipper. The next chapter, Bill dies.
