Foxy is a good guy and is just making sure you're okay. What we know is that Foxy is the only animatronic who moves on camera; He moves if you look at him too much or too little, and he's kept away from the other animatronics. When Mike doesn't check on him enough, he gets worried that something has happened to Mike and rushes to try and save him, which could explain why he runs after you, why he is the only animatronic who doesn't seem to attack when he enters the office, etc. If Mike checks on him too much, then he may think that Mike thinks Foxy will attack him and runs in to try and reassure Mike. The reason he runs, and not walks, is because if he's caught by the other animatronics, they'll try and attack him. If he finds the door closed, he bangs on it to make sure Mike is okay, and then figures that Mike is alive, as he was able to close the door, so he leaves before another animatronic sees him, which could explain why he leaves so quickly. If he manages to enter the office, he accidentally scares Mike into having a heart attack, and Mike dies from the stress, not from Foxy stuffing him in a suit, which is why Foxy is the only animatronic that doesn't grab onto Mike when he gets in the office. Now for a couple of additions that I thought up. WHY DOES FOXY CARE WHAT HAPPENS TO MIKE? He could just be a nice animatronic. He could try to save the lives of everyone the other aniatronics are after, maybe to save the reputation of the Pizzaria where he works, or just because he doesn't want them to die. This may explain the banging on the door in Phone Guy's night four message. Another animatronic was about to kill PG, and Foxy arrived to find the door to the office shut, so he banged it to try and get in to save PG, but failed. If this is true, it's also possible Foxy tried to save the kids that were murdered. If they were indeed murdered by another animatronic, Foxy could have tried to intervene, and said killbot damaged Foxy in the fraccus, which could explain his broken jaw and tattered condition. Or, conversely, Foxy is responsible for the bite of 87, which is how his jaw was broken, and he feels guilty, so whenever someone is under attack, he rushes to save them, because he is trying to redeem himself. If Foxy is indeed a good guy, that may be why he stays away from the other animatronics; because if they see him, they attack him for trying to prevent their murders. This may have happened at one point, which could also be why Foxy is in such a state of disrepair, but he escaped, because he is capable of outrunning them, and hides behind Pirate Cove until someone (Mike) is in danger, because he wants to prevent someone else by being harmed by his co-workers