I'm going to assume you want spoilers on the Sans character in specific, and not the plot of the game itself.
This spoiler tag is legitimate spoilers.
Sans is a lazy character at heart, and he's a smug comedian. He spends most of his days sleeping or agitating his brother Papyrus with bad jokes and incidental music, and just all-around being terrible at his job of catching humans.
Sans, the Lazy Comedian and Scientist:
Sans, however, is much more than the doofy pun corpse that he first introduces himself as. He has the interesting ability to simply "be" wherever he wants to be, and find "shortcuts" that lead to just about anywhere, even if the "shortcut" isn't even in the same direction as his destination. He holds some power similar to teleportation, and from examining the back room after getting a secret key from him near the end of the game, you find that he used to work with a team of people who were likely quantum physicists, and a broken down machine that many have speculated is actually a time machine. Interestingly, in his house, you can find a quantum physics book on a table. Opening it up reveals a joke book inside. Opening up that joke book reveals another quantum physics book, and so on and so forth. This is an analogy for Sans' personality, as he is both the smug comedian and the dutiful scientist.
Sans, the Perceptive:
Sans is suspected to have strong connections with a hidden character known as "W. D. Gaster" who was said to have accidentally shattered himself across time and space. Sans is aware of the fate(s) of the game world, and recognizes that the player has the capacity to reset everything. He comments on timelines stopping and starting, and in the case of the Genocide ending, he can see it all coming to an end. Sans is frightened of the power the player holds, the power of "determination," which is the power to keep resetting until they succeed. He cannot remember events that have been reset, yet still can gauge what has happened previously based on Frisk's reaction to him. He will comment on the player's deaths, and in some cases, he will comment on his own death if you've killed him previously.
Sans, your Pal:
During the Pacifist run, Sans encourages the player and provides comic relief throughout the game. He never truly seems concerned about anything, possibly because he knows that Frisk simply can't fail. He reveals to the player that, before the events of the game, he had befriended someone on the other side of a sealed door that he shared knock-knock jokes with. Unknown to him, this person was Toriel, who asked him to promise that if he ever saw a human come through that door that he would look after them. He says that if she hadn't said anything, he remarks "you'd be dead where you stand." He then says he's only joking.
Sans, the bringer of Bad Times:
If the player should take the Genocide route instead, Sans will react differently. Upon first encountering the player, he will try and joke with them, to no success. He will encourage the player to participate in Papyrus' puzzles, to no success. He will take notice of the monsters you've killed, and your LV (Level of Violence), and will remark that if you continue down that path, "You're gonna have a bad time." If you continue through the Genocide route and kill Papyrus, his brother, he will not appear in his usual spots later in the game. He will be absent, possibly mourning, until he confronts you in the Last Corridor at the end of the game. Sans is incredibly lazy, and he also knows that it is impossible to beat you. Since everything will be reset, he says that it's hard for him to give it his all. Even still, he says, "I gave up trying to go back a long time ago... all I know is, after seeing what comes next, I can't afford not to care anymore." He then proceeds to break every rule about the game's combat in order to make your life miserable. He is there to make sure you feel your sins crawling on your back.
**anonymous used "*roll picture*"** **anonymous rolled image**
The "passive" mode is the "normal" story the "game" wants to tell "you". It's really that simple, take form the quote marks what you will.
The "genocide" mode is an allegory on gamers, specifically games who have the freedom in open worlds to kill everything that populates it. AKA. . . save, kill everyone, see what happens, re-load. Though the way this game works, it forces you to save. So you will have to live with the consequences of your actions, those consequences being that you killed everyone (including the comic relief, sans, who is both the strongest and weakest character). Just like that time in fallout 3 when you killed everyone in megaton, you monster.
were it true or not, it is something expanded enough on the internet to have no way back. you can see a skele ton of videos with that non-canon part of the story with that song