[The Squatter of Amon Rûdh]

Actually Mîm was killed by Húrin, who ran into him in what was left of Nargothrond after his release by Morgoth.

What the Petty-Dwarf had done essentially was to betray Túrin and his outlaws by leading Orcs to the hidden entrance to his halls on Amon Rûdh, where they had set up their base of operations. This is understandable since Túrin and company had killed his son, Khîm and taken over his home. Unless my memory deceives, Húrin killed him for speaking out of turn. He was more sinned against than sinning in my opinion.