CREDIT TO DOOMY BOY
Patrick was a legal human citizen of the monster empire. He was as respected as a human could be in the eyes of monsters, but he made the mistake of trying to play both sides during the war. Offering to help the monsters keep watch over the human prisoners of war, Patrick would secretly help smuggle them back to the Human City. The Royal Legion, having noticed that many of the prisoners Patrick reported dead had no corpses, discovered his treachery. The Greatest Dog ripped him to shreds, but did not kill him. Instead they turned him over to Gaster. Killing him would have been more merciful.
Gaster chose to extend Patrick's suffering for all eternity, stitching what remained of his body and his soul into a scarecrow. Then he nailed said scarecrow to a post, leaving poor Patrick, or as he was now known Stitches, in the middle of a field, intending to let him hang there and starve to death. That should have been the end of the story.
But it wasn't. All the experiments Gaster had done on "Stitches" had turned him into something neither man or monster. Now he waits for anyone, monster or human to pass through his field, and he murders them.