I like to think I'm a nice person, but when I was a teen I was really into nihilism (still kind of am) and not very big on social subtleties, and empathy etc.
So, I once told my classmate that they didn't matter, and that their life was insignificant and that in the great scheme of things they would never amount to squat.
Thing is, I didn't even mean it maliciously back then. I just really thought (still do) this about everybody, myself included.
Needless to say, their friend who was nearby lashed out at me "are you stupid?!" (and I guess I was, when it came to understanding others, though I like to thing I got better. I still feel like an alien sometimes though) and gave me a mean look.