who's to blame? kaito i guess?
Miku, I'd say. Not in terms of the promise, since she honored that, but for how she handled things. She straight up tried to murder Luka. You can't hurt people because you didn't get your way, plain and simple. Kaito and Luka handled the situation well, honestly. They were direct with Miku in the end. Unfortunately, she was already over the edge by that point.
The promise to not pursue a relationship was in vain. Feelings can persist and relationships change over time. While a competition for his interest would've changed the group dynamic, it would've set the tone and (maybe) lessened the damage to Miku's psyche.