In the state of Colorado, a serial killer is caught. When they search his premises, they find a small grow-op. A whole debate ensues about whether or not marijuana causes people to kill. That's the backdrop.
The main story is that his weed gets out on the street (crooked cop who sells it, or someone steals it), and this weed has the magical power of putting the serial killer's personality inside the smoker, thereby turning them into serial killers as well.