I feel like if Basil saw Mari accidentally kill Sunny, he'd probs feel a bit more resentful and angery at her. I hc that Sunny confides in Basil a bit on his sister's perfectionism and the brutal practices she's (unintrntially) making him do.
Since Basil is very, like, devoted to his best friend he'd be distraught at the fact that he couldn't help him before it was too late so it haunts him for the rest of his life, especially if he ran away and doesn't do anything and just ghosts everyone. On top of that he if he ever reconnects with the others he might have a very strained relationship with Mari and probably resent/hate her for Sunny's behalf, cuz that's what Sunny would do right? If his sister forced him to perfect his practice for a recital he never wanted to perform in, and then killed him when he wasn't good enough for her standards?