Sans fled to Snowdin after you fight him in the ruins, he started his second killer spree, gaining alots of LV while you get less LV, becus sans killed most of the monsters, you killed the rest

You encountered him at the edge of town

Sans: looking at you it's you... again...

Sans: why don't you stop following me...

He looks more stronger

Sans: do I really need to fight you again, I needed to kill my brother to make me more powerful...

Sans is prepared for A Rematch