Book and Switch: This is how Tom Welles realises Max California might be useful. He's reading porn while working in an adult bookstore, but Welles notes he's underlining passages and calls him on it. Max is actually reading Truman Capote's In Cold Blood.
Defensive Failure: Welles has one of the men involved in the making of the snuff film tied to a support bar in the building where the film was made. He has his gun trained on the man's head...but can't bring himself to fire, even as the man taunts him (to the point of licking the gun). Welles then steps outside, calls the mother of the snuff film's victim, and begs for permission to kill the man. He gets it. Five seconds later he pistol-whips him to death.
Disappointed By The Motive: In the climax, Machine admits that he killed the girl because he simply wanted to, and that he likes to hurt and kill people. This enrages Tom enough to kill him.
Divide and Conquer: Welles is about to be murdered when he reveals exactly how much money — a million dollars — Mr. Christian paid for his snuff film. The others immediately turn on the lawyer who commissioned the film, giving Welles time to grab a weapon.
Dragon Their Feet: Welles kills (or sees killed) the people who created the snuff film before going after the "performer" who murdered the girl. Justified, the actual murderer was injured and it took Welles a while to figure out he was.
Driven to Suicide: When Welles informs her the film is real, Mrs Christian kills herself rather than live with the knowledge that her husband was responsible.
Even Evil Has Standards: When Tom first investigates the underground porn rings for snuff films he is driven out at gunpoint for daring to ask for it. Others into extreme bondage rape and sell kiddy porn are likewise angry at being asked about it. It takes a real sicko to find one, and even then it's fake (simulated, no one dies.)
Impersonating an Officer: Welles poses as a government investigator to interview the girl's mother. As he's working for a highly paying client, I guess he thinks it's worth the risk. Toward the end of the movie, he claims to be a police officer when ringing around the hospitals looking for the man he stabbed, but by that stage Welles is so personally involved in the case that he doesn't care about the legality.
Improvised Weapon: His pistol having been confiscated, Welles stabs a villain with the pistol's cleaning rod.
It's Personal: Welles is eager to solve the case, but only because he believes it will give him an entrance into the lucrative world of high-society clients. By the time the movie has finished, he's graduated to full-on Vigilante Man.
Karmic Death: Machine usually uses a sharp long knife to kill his victims. At the climax Welles kills him with the same weapon.
Mood Whiplash: Welles and Max finally stumble upon a purported snuff tape which shows hardcore sex followed by a grisly, real-looking murder of the victim. Horrified, they skip forward... and find that the slasher's next soon-to-be 'victim' is the same woman. Cue relieved, if disappointed groaning.
Mysterious Watcher: He turns out to be the attorney Daniel Longdale, who commissioned the snuff film for Mr Christian. While it's odd that surveillance expert Welles doesn't notice him, Longdale has the advantage in that Welles is reporting everything he does back to his client, in order to show how hard he's working on the case.