Spoilers from Mikkos Route! AND SORRY WRITING SO MUCH
A lot of people seem annoyed at the "Bad ending" of Mikkos Route, but I think I have somewhat of a different take here, I personally don't like to attach to the past, I find it kinda toxic, so when I saw that an old best friend was out there in the romance options, I just avoided him and played every other route first and left Mikko for last, and as I got to know him first by the others routes, It kinda creeped me out how petty and obsessive he seemed to be, he's fixated on us, in a unique toxic way, and I kinda disliked him for that.
So when I played his route I couldn't help to see the red flags of someone who felt powerless, he had to move from home because of you, he lost all his friends because of you, and then as soon as you come back, he changes himself to please you, (you can even see his inner thoughts, he's really obssesed to please you!).
And from what I get from all the other routes, is that he didn't move on from you after all this years, and even if you didn't either, at least you're trying. He remembers every single detail of those days with you because he wrote all of those same details on a diary that he STILL reads years later, repeating all of those events like mantras, and despises you for not remembering those days as well as he does now. I was SO creeped out until the end.
And... since I played all the others routes first, I dettached from him as a character, but if you do that, you can start to perceive all this toxic form of attachment he's formed with the idea of you.
I really like the "Bad ending" because it makes a lot of sense for his character, you're kinda the Torulf on his life, and by choosing to re-attach to Mikko instead of moving on with the other routes and characters, you're awakening all this trauma that he had to endure, bringing the worst in him.
In my personal opinion, that's why both of his endings are bad, because the only way of giving HIM a good ending is by not choosing him on the first place.
Spectacular writing! Kudos to the creator for giving us the opportunity to get to know a character through other routes, instead of just writing him off or ignoring his existence, as many other writers would do.