In reality, the pause was almost certainly due to the turn. I have no idea if he was justified in shooting at Brown while he was fleeing. However, that would not get him charged with murder in itself (rights violation maybe under Tennessee v Garner assuming Wilson couldn't come up with a legitimate reason for shooting at an unarmed fleeing suspect). The murder charge has to come from the reason he fired after Brown turned to face Wilson. If Wilson shot Brown without some sort of sign of threat from Brown, then it justifies murder charge. However, if he shot him because there was advancement toward him or movement indicating that he, Brown, was going for a weapon (Wilson did not know whether Brown was armed or not, even if he got a description of the robbery including that there was no weapon used in that incident because a police officer cannot assume that a suspect is unarmed just because they did not use a weapon in an earlier situation), then that is justification for an officer to shoot a suspect, and police officers shoot to kill, not wound. As of right now, we don't know which it was.