Other mistake: When they crash head on into the gang's car before robbing them, in the brief shot inside showing the impact Mel Gibson's head whips forward as you'd expect, while his passenger's head doesn't.
Other mistake: When Porter cuts the fuel line on the car and is about to light it, Carter's henchmen see him and act surprised. Despite having ample time to get out (as any rational person might do), they just sit in the car and wait for it to explode.
Answer: Quote, "A work man is worthy of his hire." In the original version, the Mob Boss asked, "why are you doing all this?" He replied, "I want my money." It's as simple as that. He did a job and wants his payment.