I think what had happened was just a whole lot of foolishness and apathy.
res thought it's funny to read people's PMs. That's the entire reason. res is a fool who doesn't quite understand that actions have consequences, and that maybe it should consider other people's feelings before doing something, maybe. If it makes anyone feel better, I don't think res or Mai ever accessed anyone's emails or other accounts outside of TCoD-related things (various BfA forums and TCoD itself), so you don't need to wonder about stolen credit card information or anything.
As for Mai, Blazhy, and Allie -- they just kind of ... went along with what res did because they enjoyed hanging out with res, and it's easier that way. Blazhy didn't have enough initiative to report, and she had cared enough about her TCoD friendships to try to keep them (by lying, not her finest decision). And allitersonance considered reporting, but decided that she didn't really care enough about TCoD or ASB to do anything about it (she also says that she didn't realize that res had been stealing passwords until recently.)
In the end, it's just so easy to go along with what your friends when they do something bad. It's so easy to rationalize it away. Admitting that your friend might be not the good person you thought they were is hard. Taking action against that your friend is even harder. And so you go along with them, not really thinking about the impact of your actions until everything comes crashing down on 2.5 years later.
With this in mind, I would like to post one of my favourite quotes:
"It's better to be a lion for a day than a sheep all your life." - Elizabeth Kenny