Other mistake: When Benji kills Wistrum, the bullet hits him square in the back, even though Benji shot him from the side.
Other mistake: When Ethan drives a car off the edge of the parking ramp the car falls 100 meters to the bottom pointing nose down the whole way and hitting nose first allowing Ethan to be saved by its air bag. The car's angular momentum makes this impossible - the car would have tumbled back over front.
Other mistake: During the car chase scene in Dubai, Ethan pushes the clutch several times (visible in the leg-room shots). The car, however, has an automatic transmission.
Answer: He considers Hunt to be a friend, so did what he asked. In addition, by carrying out the deal he has ingratiated himself with Sidorov as well.