After Kaeya died, we sat down as a group. The five of us from the same game. Hua Cheng asked all of us if we would defend ourselves if we got attacked on a Thursday night. Diluc and I both said no. I dunno Diluc's reasons, but I've always been saying mine: unless a shadow forces me to, I won't kill anyone. Never again.

So...I guess if he thought Sylvain would attack either of us, that'd explain why he would attack Sylvain.