So if a person with BPD would discover LW and decide they like the ideas, what would they most likely do?

My model says they would write a lot of comments on LW just to prove how much they love rationality, expecting a lot of love and admiration in return. At first they would express a lot of admiration towards people important in the rationalist community; they would try to make friends by open flattery (by giving what they want to get most). Later they would start suggesting how to do rationality even better (either writing a new sequence, or writing hundreds of comments repeating the same few key ideas), trying to make themselves another important person, possibly the most important one. But they would obviously keep missing the point. After the first negative reactions they would backpedal and claim to be misunderstood. Later they would accuse some people of persecuting them. After seeing that the community does not reward this strategy, they would accuse the whole LW of persecution, and try to split apart their own rationalist subcommunity centered around them.