The Minus Man
(1999)

Owen Wilson is a quiet serial killer who poisons his victims.

While on the road, he poisons a bar fly (Sheryl Crow) for no reason.

He arrives in a new town, and rents a room from Brian Cox who suggests he find seasonal work at the post office, which he does.

He periodically sees imaginary police men played by Dwight Yoakam and Dennis Haysbert, who might represent his conscience.

He poisons a high school athlete for no reason.

He attempts to connect with a coworker, Janeane Garofalo, but he is too detached to succeed.

He poisons a patron at a diner, eating alone on Christmas day.

The landlord's wife (Mercedes Ruehl) is murdered and the police come to his home and arrest Brian Cox. Fearing scrutiny, Wilson leaves town with no hope for justice or understanding why he kills.

Thanks Don P!