It seems like he did have some things going on with himself. He also just seemed like someone who's prone to sadness.
They had both already gone through so much together. His fight with Mafuyu was just the final lil push he needed to go over the edge. It's heartbreaking.
crap this broke MY heart
Mhmm... Like, it was obvious how much they loved each other and how much they depended on one another. It's normal and ok for Mafuyu to still be hurt over losing Yuki. It's also ok for him to start moving on and find new love. I think Uecchi is a good and supportive boyfriend-- even if he can be a bit of a drama queen when it comes to the band. I'm glad they are together.
Did Yuki have depression or some other mental illness? I just don't get why he did what he did over a fight with his boyfriend.