My main issue is that MC's entire way of thinking has done a complete 180 over night; he's gone from a generally nice, moral, well meaning person to a literal sociopath in a ridiculously short amount of time. Behavior in-and-of itself is subjective and can shift radically on a whim, but the thoughts behind that behavior are another thing entirely; the way a person thinks takes time to change, and we've had a front-row seat to the way MC thinks for a route and a half by now. It'd be one thing if he was fully aware of his behavior and acknowledged it for what it is, but he isn't and doesn't, which is the crux of the - or rather, this - issue. Unless the behavior is qualified in real-time in MC's thoughts as his way of acting out and distancing himself from his feelings for Tai (e.g. "Tai clearly didn't care about me so I won't care about him" or "I'll just get with someone else and forget all about Tai"), there's no way to retroactively justify the way he's acting while in the first-person perspective.
Not even to mention the fact that we, the player, were showed the entire conversation between Tai and Spencer, which only served to justify the way Tai responds to MC's confrontation and further cementing in our heads how douchebag-ish MC is acting, as we have the full context. It would have been better to leave us in the dark about the full context of the conversation and, instead of Tai's unnecessary confession, leave the conversation off on a vague and suspicious note, something open to interpretation that could reasonably leave MC with some doubts; this, paired with the ring Tai was - by MC's perspective - suddenly waring around his neck and his blatant refusal to share any information regarding his past with Spencer, would bubble over into the situation we now find ourselves in. The way everything is executed now is just cheep and for the sake of setting up a dramatic act 2 climax, and the story pretends that there will be no way to reconcile between Tai and MC as though we didn't set out to play Tai's route in the first place; it's a series of terrible tropes that stick out like sore thumbs in an otherwise well-done story.
Consistency is key with characters in this kind of medium, and conveying a person as flawed is not the same as turning them into a manipulative sociopath when they've showed no tendencies towards such behavior before. The way MC is thinking right now completely flies in the face of almost every conversation he has with Tai in Diego's route; we have the perception of MC that we do because we've already been through an entire route in his head.