A super-secret government super-soldier hitman, grown in a lab with superhuman strength and regeneration powers and programmed from birth1 to be a mindless killer, goes rogue after watching some porn or something.
He has some psychic link compelling him to pursue the man he is cloned from, an ordinary civilian citizen due to a mixup. He is like a psychotic evil twin of that citizen.
Due to psychic connection causing unknown terror, the citizen flees across the country with his family. He also had an irrational urge to flee to the West.
After a pursuit across the country, they meet in California, maybe. After recovering from complete cranial destruction by gunshot wounds by eating 20 McDonald's Happy Meals, the assassin tries to kill the whole family. The assassin's government handlers try to handle things.
Chronology and other relevant info
I would guess the novel was from the 90s. At least I think that's when I read it. I thought it was someone well-known, Koontz, Crichton etc. However I couldn't identify it from any list of their published works.
1. He could possibly be a clone-born.