This is a Community AU, it has no story or anything, but Underswap!Sans just randomly turned into a yandere. But this AU is popular because of that.
A yandere is where a person was obsessed with their lover that they kill people who they talk to, or who they are friends to. To prevent them to "taking away from my senpai". This is commonly used in Girls. ( Well duh.. ) that's what Underswap!Sans is like, again, he is a yandere. Who is his lover? Either Chara/The human/You or Underswap!Papyrus. He is always seen in fanarts trying to kill Fell!Sans.