I like Vic Mackey, but he's a bad guy. A bad guy with good tendencies at times. But when the Shield started, and throughout it he has at times been in bed with criminals, stolen money, and used all kinds of illegal tactics. He exists somewhere in a gray zone now, but it doesn't excuse the crimes he has committed on the show, or in the past.
Terry was doing his job. Reporting rogue cops, which the Strike Team were. How does that warrant his murder? If anything, he was the one truly doing the right thing in this.