McDonagh uses his acquaintance with Big Fate to plant fake evidence at the crime scene. Isn't the police now bound to find out about McDonagh's secret deals with Big Fate? Would they just forgive McDonagh? After all, he gave out police inside information to the crime world. Surely Big Fate, Midget and "G"note *derisive snicker* will use every opportunity to implicate McDonagh? What if they all indepently testify that McDonagh planted the evidence? — Yet we only see that McDonagh gets promoted to Captain. How did he cover up his own crooked machinations?
Who are the cops going to believe: a known drug kingpin who just got arrested for a multiple murder, or the decorated hero cop who busted him? It's his word against the crooks'. No one's going to believe them.