What you don´t get or don´t want to get about Emma, is not if Seymour is an antagonist in ORS or not, but the way she operates in general! She has no proof, just a feeling or idea. But this feeling, which could be fully wrong, convinces her she is completely right and simply follows this, obstructing and attacking someone only going by this feeling!
This is not about Seymour, who might be or might be not in murky business, this is about what she would do to anyone! If she would get the idea that Lena or Ian or Joe Doe or Jane Doe are into something she does not like, she would do to them what she does to Seymour, no matter if she has evidence or not!
That is my point here, Seymour being a maybe antagonist of the game, just glosses over this behaviour. I know people in real life thinking like that, you do not wanr them to be your friends! Because for them, only their opinion counts, no matter if right or wrong.