A meme in and of itself is not destructive, what causes the destruction is if someone takes it and overuses it or already hated the subject and used it to cause destruction. In real life things like memes happen but because of the smaller number of people the chances of one of those happening is smaller. On the internet because it consists of everyone then if say one in a thousand people wanted to use a meme destructively you would have enough people to go critical and start a death spiral for the subject of the meme. The more people that like the subject the more people you need to destroy it. Having more memes about the same subject lowers the critical point because of intermeme conflict.
Also things like "brony" are not memes, they are descriptors. Because of the internet such things get a real work out in their definition. In real life its easy to say for instance that you are an American because you live in America or that you are Australian because you live in Australia. On the internet most groups are formed by what you like and where you frequent so all of us who frequent Bay12 would have a name for ourselves but because it is not related to any specific thing outside of likes and such there is no real way to prove who truly is one or not.
Which brings me to another thing about why internet memes tend to be destructive. There is no real way to say that someone is not what they say they are. If some random idiot somewhere calls themselves a brony and starts dropping memes about MLP you have no way to say that he is a Troll. In a real world group you could easily point to someone who is causing trouble and thus easily make them leave the conversation. Sort of banning which is relatively easy to get around if you want to the internet has no way to remove dissenters. Even if every other person hated MLP with a passion a real world group could easily keep itself together just my not letting anyone who doesn't like it into their group. Online having half the people hate something would make it be treated even worse then the furries are.
To wrap up I will say that memes can be destructive if someone wants to use them that way or uses them to much. Because of how the internet connects so many people the chances of this happening are increased. Ergo most internet memes tend to be destructive.