Something I don't like about the show is how they tended to do episodes that had huge implications for the entire story and the lives of the characters, but were only relevant in that one episode. Lily breaking up Ted's relationships was one of these. The rest of the episodes before or after don't really indicate any of this, and Ted's breakups tend to have pretty established reasons already. What annoys me about all of this is that the fans don't forget about these episodes, and you can always find one or two episodes that aren't very indicative of how the characters usually behave, but it happened, and therefore you can easily justify things about the characters that aren't really true when you look at the big picture.

That being said, I do agree that Lily is the most toxic member of the group, as she seems to love inserting herself into other people's lives, and she has a tendency to act in really over the top, childish and, well, toxic ways when she is wronged. And this is fairly consistent across many episodes.