In all the ways that Dan was a demanding, downright evil parent, his older brother Keith, played by Craig Sheffer, was the exact opposite. Keith supported Karen when she was a teen mom, acted as a father figure to Lucas, and (usually) refused to engage with Dan's taunts.
Keith acted as a mentor to many of Lucas' friends, including Jimmy Edwards. Over the course of high school, Jimmy became more and more troubled, and during season three, he brought a gun to school. Keith, who knew Jimmy, went into the high school and attempted to talk him down before he shot anyone else, but ultimately Jimmy committed suicide in front of Keith.
At this point, Dan entered the school, and in a jealous, paranoid rage — Keith had slept with Dan's ex-wife Deb and Dan was also convinced Keith had tried to burn down his car dealership with Dan inside — murdered his own brother.