Mitsuru is a narcissist. He expected Sakiko to sit still and cry for him, but she didn't isolate herself from other people like he hoped. Taking a jab at Sakiko and Itsuki was a low blow. He's just angry that she moved on and defends another man instead of him
Truth to be told, if I were in an ML position, I would've called a mental hospital. Imagine someone showing up in front of you claiming to be from a different era