But the whole game you run along and play ball to satisfy Haines & Weston.
If Mike & Trevor are really so pissed and dislike them, why they don't kill'em right away?
For Michael, doing what Haines wanted was a way to ensure the FIB get off their back, and while Weston was a bit of a dick, he did introduce Mike to Solomon. It's only in the aftermath of the Meltdown affair that Weston becomes a real threat.
For Trevor, doing the FIB's dirty work was one way to try and get Brad out of prision. By the time he finds out that Brad is dead, he is finished doing anything for Haines and Norton. Weston was supposed to be a payday for Trevor.