BoBoiBoy doesn't smile. He doesn't quip. He looks at his own hands as if they are foreign objects. He whispers something (if you listen closely with headphones, it sounds like "Maaf..." - "Sorry").
It is a kind boy who has run out of kindness. BoBoiBoy VS Borara
The Context of Cruelty To understand why this fight is so profound, we have to look at where BoBoiBoy was mentally before this moment. BoBoiBoy doesn't smile