I know some people who are absolutely harmless in person but absolute a*holes online. You often have to picture them to understand the animosity online. And this is not just in 1-1 situation, it can be in large community posts.
If you met them in person, you realise they're physically completely harmless, you could even classify them as 'weak'. This visual / physical context is everything that is lost in online communications. One perceives someone as a threat using a vector of things: online, we lose most of that frame of reference.
One overlooked element is that, online, people lack of empathy and understanding for their audience. Part of communicating effectively is your ability to see yourself through the shoes of your audience. Online, this is lost since you can't picture your audience, so you have to act like it's the lowest denominator; and explain yourself really clearly and politely in order to pass a clear message (being well understood).
Most people, especially younger people I find, completely lack the social skills of writing online to strangers. Not only they're unable to picture their audience or put themselves in the eyes of the reader, but they seem to not give a flying rat about it in the first place, leading to two very different persona: in-person, shy, quiet, reserved, and online, loud, insensitive, inconsiderate belligerent idiot.