Remember George Sodini? He was from around here, in Pittsburgh, and one summer day about two years ago he went to an LA Fitness with a duffel bag full of guns and shot up a women's aerobics class. His journals were full of racist and deeply misogynistic ramblings. He had rage and entitlement issues. And his whole life, women would say he was a "nice guy" while they rejected him.
And, yes, he was well groomed and polite on the surface, but obviously had some problems. The women he hit universally stayed away for a reason.
Read this article about it by Kate Harding. Here's a taste:
Telling a guy the real reasons you're not interested -- you don't find him attractive, he's way too old for you, you get a distinctly creepy vibe off him, whatever -- or offering no explanation at all, because you just met this guy and owe him nothing, would be "rude." And thanks to the conditioning Harriet describes, exhibiting the slightest hint of "rudeness" to any stranger who approaches you with sex on his mind makes you feel not like a normal human being with healthy boundaries, but a mean, frigid, stuck-up bitch. Worse yet, sometimes, the same man who called you beautiful and offered to buy you a drink ten seconds ago will turn aggressive when you say you're not interested; he'll tell you flat out you're a bitch, or a whore, or less printable things. He'll reject your rejection by getting in your face and losing his temper. So really, it's a lot safer and simpler to say, "Look, you're a nice guy, but no thanks/I have a boyfriend/I can't." Most guys will walk away calmly after that -- and hey, it's none of your concern what they go home and write in their diaries.