Question: What was Mickey's motive for being one of the killers?
Answer: He hadn't no motive. Remember? It was the millennium. Motives are incidental. That and he was out of his mind. And he wanted to blame the movies.
Answer: Mickey's reply when Syd asks, that he's "always been a follower," indicates he's probably just psychotic, looking for any excuse to kill.
Answer: He wanted to become infamous by going to trial and blaming violent movies.
Answer: His main motive was fame, or infamy for that matter.