After the Greatjon dies during the War of the Five Kings, the Smalljon becomes the new Lord of the Last Hearth. He betrays Osha and young Rickon Stark to the Boltons because he hope that they will help him fight the wildlings. He despises the Boltons and refuses to show them any respect. During the Second Battle of Winterfell, he is killed by Tormund Giantsbane who rips his throat and stabs him.
In the books, the Smalljon is not villainous at all, and in fact, he dies during the Red Wedding while still on the Starks's side.