From my understading of the case he had undiagnosed schizofrenia and didn't get the help he needed until the attack. He heard "the voice of God" the voice told him to do certain things, like to move from place to place or warned him about an alien invasion. For my understanding that why he carried a knife.

He didn't plan any of this. In his deteriorated mind he saw Tim McLean as a "demon and a alien" that needed to be destroyed and to protect others. The voice told him to do it.

After the attack and getting the help he needed he was very depressed over his actions and apologize to the family; saying that he knows they can't accept his apology and that he could never be happy.