LOL, watch the movie again. It might help you. If I recall correctly, Captain and his friends were killing mobs to take over the drug control. That's where the $$$ is. Lt. (Guy Pierce) knew the police dept. and city concil were crooked to the roots and he was a political animal. He took the promotion and became C.O. of Homicide division. I think he took the job because you can do more at that position than trying to fight the corruption.
Just my opinion here.