When an artist receives criticism, they take it to heart. If that criticism contains advice on how to improve, the artist may fix the problem that sparked the criticism.
If that criticism is something to the effect of "You suck. Your art sucks. Take this shit off my internet," then the artist isn't going to get better. In fact, depending on how sensitive the artist is, he or she may actually comply and take down their work, and may even stop creating new art for a time.

The latter form of criticism may be considered a form of trolling. However, I have seen CREATIVE forms of trolling, such as recordings of organized micspam, hacking and such on games. Granted, that kind of thing is still low and annoying, but it is something that can be seen by masses of people - and as such, it is LEAGUES ahead of posting hateful comments on sites that nobody but a select few will ever see.
Such limited-viewing criticism is far better classified as bullying. Its only effect is to belittle one person; the bully will thereby feel that they have gained power. But let them just try using that "power" to get out of paying their bills, and they'll see how worthless it is.

I have seen this cyber-bullying before. Thankfully, I've not been on the receiving end of it, but having been bullied and teased throughout elementary school and Junior High, I know how deep words can cut. Sticks and stones can break bones - bones can heal. Words create infections that can fester throughout life.