I don't think it has anything to do with you or your experiences actually, I don't know you. I'm merely pointing out the danger of assumption and deciding your perception of another person's intentions is correct.
I'm not quite sure I understand what you mean by this: Why is it different if it is posted on someone's RP comments as opposed to someone's personal blog or youtube?. Because the difference, clearly, is that RP comments are characters speaking to each other whereas the opposite is, well, a person attacking another. These two things couldn't actually be more different. I think assuming that the person leaving the comments is doing so as a personal attack is, actually, doing that person a discredit--because you're jumping to the conclusion that they are uncreative and not capable of separating 'Real Life' from 'A Game'. I'm not talking about comments left on a CDJ, comments left on fan fiction or posts in RPvents. There is a difference between talking to a player and characters talking. There is a difference between one character attacking another and one player posting nasty comments about another, because there is a chance that these things are being done in character, no matter what you perceive.
And no, assuming along doesn't make you guilty of bullying. But assuming and doing something about it, like posting about the assumed bully on RPvents certainly does make you guilty of being a bully yourself, I think. I think there is a very delicate balance between actually being a victim and victimising someone else when you're acting on what you perceive and not what was intended.