Lucky Luke 1 Billy the Kid

Lucky Luke is a one-hundred-percent real hero of the Old West. He is squarely on the side of the law and dedicates his life to seeing that justice is done. He travels around delivering it wherever he goes, accompanied by his faithful companion, Jolly Jumper.
Arriving at Fort Weakling, he changes into a formidable bandit. Why?
Does his meeting with Billy the Kid, a living legend of the Old West, influence him?