Calling someone by their first name doesn't make them seem as threatening, plus it conveys a lack of respect for the person.

It's like referring to your boss or someone by their first name instead of "Mr. So-and So." It conveys a level of familiarty inappropriate to your relationship, as well as making them seem (in your mind) just like you by tearing away all those honorifics.

In my line of work, officers are always (well, supposed to be) referred to by rank and last name by those they outrank; however, they can call you by your first name (there's a whole realm of material to explore in just that fact). If Col. X is an unrepentant asshole, most people will call him by his first name behind his back. (ie, "Okay Bob, I'll get right on that.") Doing that strips away the poofery and aura of authority and makes him seem like a regular human being no better than yourself.