Eventually, the Bobby view of cops is replaced by a statist view. Statists view the government and its agents as the good guys. The bad guys are everyone else. It is part of a cop's routine to invade the rights of citizens, earning their hostility. Ordinary citizens are a deadly danger, like a powerful beast to be kept chained lest it run amok.
Armed citizens are the most terrifying threat a statist cop can imagine.
Gun control is a watershed issue by which to judge the moral character of a cop. The officer who still regards himself as a moral agent defending the rights of citizens will oppose gun controls. The one who regards himself as the state's hired gun will support gun controls. The police establishment has made its support for gun controls perfectly clear. But Joe Cop is silent, perhaps from fear of subtle, vicious, career-ruining, establishment reprisals.