Space Marines do what is best for Space Marines. They have a big interest in seeing the Imperium servive since they life within the Imperium and they get there recruits from us little humans. Without the Imperium the Space Marines do not exist so if they want to survive than they will usually be helpful if it suits there interests. When Space Marines help a Warmaster than they more or less earn a favor from that Warmaster. All of this leaves the Inquisition from questioning there worth.