I am unhappy with this development but judging on the amount of chapters left, there will be reconciliation and it will be explained later (dont believe this is the type of story that will go into the washio is brainwashed with magic thing).
Its obvious that Washio rejected her so brutally because in his mind he thinks they all would be better off without him. He believes that them meeting him again put them in undue danger and he puts their happiness above his. Brutal rejection will transform her love into hatred, and one might believe that tidying up this matter like that would allow for swifter recovery from unrequited love. Sacrifice for the people you deeply care about is worth it, and Washio's own mental health issues prevent him from seeing the more favourable conclusion (guilt combined with the fear of happiness I assume). Perhaps Washio might even feel that he had no other choice but to do this.