Bobby Lee is the guy who smiles noticeably and apparently thinks killing people is "some fun." He's one of The Misfit's two accomplices. He's young—he's described as a boy—though we really know how old he is.
He does seem to enjoy killing people; he's "grinning" all the time, including when he grabs June Star and takes her to meet her end in the woods. He also thinks that shooting the grandmother every moment of her life would be "some fun" (141).
If The Misfit is a more complicated criminal who might have undergone a transformation at the end of the story, Bobby Lee certainly isn't.