Board Thread:General Discussion/@comment-37345491-20190406164451/@comment-37345491-20190603183800

Asthonman wrote: While Sans doesn't develop much as a character throughout the game, I think that's largely because most of Sans' character development happened PRIOR to your arrival. He discovered that his world can reset without any warning at the whim of a single person, and fell into a state of apathy PRIOR to you falling down the mountain. He made a promise with Toriel to look after any human that exits the ruins PRIOR to your arrival.

On the subject of the promise, I think Sans' statement that if it wasn't for the promise you'd "be dead where you stand" implies that he may have a prejudice against humans, which would make that promise a big step in character development. The fact that he develops outside of the game and not during the game is not an excuse. If he doesn't have any development in the actual game then it doesn't really work. Promise or not, I don't think Sans has any motivation to harm you because he knows the timeline will just reset anyways. He only fights you on Genocide because you killed everyone and that is his motivation.