Ok, so I have mixed feelings about this. Before I would have thought that what he did was perfectly justified BUT after reading the first chapter its not so cut and dry. First of all, leaving the relationship was his choice and he had to do right by himself, especially after being abused day in and day out, not questioning that. She turned into a complete bitch and no one deserves to be treated like that. THAT SAID, she was obviously a good person at point, and only treated him like that because she was truly and safe comfortable with him when she was under tons of stress, and he understood the stress part.
So this is where I run into a dilemma. Relationships are a 2 way street. If there are issues you try to fix them instead of just walking out immediately, which he didn't seem to even try, at least not that we even see. He instead snuck out on his own to do quests, instead of being supportive of her work, which led to her being more stressed and treating him worse cause of it, which again, he understood. So honestly what he did was in essence emotionally cheating and leaving her stranded with all the responsibility. Then he peace's out, leaving her distraught without truly understanding why, because while she did think she may have been a little harsh, she didn't realize how bad she's gotten.
As I was typing this I got more and more mad at the MC realizing the whole situation. His girlfriend was the pushy type that was a busybody but had his best interest as heart, just happened to get too much responsibility pushed on her without a healthy way to relieve it. While the MC is, and much as I don't like the word, a beta, that kept internalizing everything without putting in any effort to fix the relationship till he decided it was time to screw off. Overall it was a mismatched pair.
If we are placing blame, I would say that its at least 80% his fault and 20% hers. And at first I really felt bad for the MC, but after thinking it over, no, he is very much at fault here for being a crappy person and boyfriend. Also I now realize that this is probably the author self projecting about a bad breakup or something, or what he thinks his audience will rally behind.
What I hope will happen is that the girlfriend will find a person that appreciates her and helps her get out of the rut she is going to be in. And that the MC gets payback for being a piece of crap person.
But what will probably happen, is that the MC will get a harem of girls and have the girlfriend crawling back to him asking for forgiveness, and will either get it and be accepted into the harem after some degrading events for her, or be told to f off. I really hope it doesn't go this way, but we will see.