I think it's all going to come back to glasses girl. She's clearly friends with Masaki, they've possibly known each other a while. We know Kazuya screws her up emotionally during/after their relationship. My guess is that Masaki tries to pick up the pieces with her, but ends up failing in the end, and she kills herself. He then blames Kazuya for it all which leads to the strangling. Would explain why he gets premonitions when he gets close to any girl, as the only good ending would be responding to her feelings.
I'm also wondering whether Masaki is his older self as well or not. Lots of times I feel like it, when he gets his death stare going and knowing about Yoshino's boyfriend. Not sure his actions overall really fit in with that though.