Okay, let's build on my little tower of guessing blocks here. Ye or whatever the fuck saw a fate of himself and Yotsuha together at an early age, and it drove him mad. He became obsessed with her and got jealous when she was getting close to Uichi, and then some other unknown entity decided to push him over the edge by thinking he needed to have Uichi killed off to get what he wanted, and that's all he needed to do. Someone theorised it's Uichi's mother, but I don't think she's that cunning, considering her mental state. Seeing as there's a possible element of time travel hinted at within this story, my guess is something supernatural, like maybe a goddess disguised as some dark-haired bitch looking out the window,
cough, cough