(Spoiler) Can anyone explain to me why....

Because they had killed the two gang bangers on the lieutenant's orders, and he was afraid that they were going rat after they drew all the heat for killing the innocent old guy....which turned out to be the case, as we see Clifton talking to Internal Affairs in the beginning. But unless the lieutenant knew that Clifton was talking, I don't see why he wouldve turned on them so easilly if he trusted them to do the hit in the first place......(I think I might've missed something about word of Clifton talking leaking. When they're at the safe house and the two other cops come to kill the main characters, the two would-be killers say that they were sent because the lieutenant was afraid of the main characters ratting. Cuba gets all deffensive, but Clifton turns away ashamed. Maybe Clifton's guilt hits him, or maybe the implication is that the lieutenant got the idea of them ratting because he found out that Clifton actually was.....sorry if that was rambly and muddled...)