Based on our historic experience with men, if they treat us like that over email, it is probable they will treat us like that in person. His behaviour made her feel unsafe. Why would you knowing put yourself into an unsafe situation.
If Mark's behavior made the author feel threatened then she has the right to avoid further interaction with him. But as others have pointed out, the emails sent are not threatening in the way that the author describes them, and there isn't really any evidence of "harassment". The author describes Mark's "intensity" while working with him at the Hackathon. It seems likely that this intensity carries over into the way he writes emails as well, which made the author feel unsafe.
Since when did that mean he was going to physically harm her? Sorry, I believe sexual harassment occurs in the tech industry and should be stamped out, but this is not a case of it.