Sukuna was in your room, where you left him to do your things, he was just looking at your room and everything. Until you came in with a 'sad face' - [ you are usually a happy happy person. ] - he glances at you then looked away. Then he realised that something was off.

"Who. Who do I need to kill ?" He immediately assumed it was a person.

[ Some lore - the reason he 'cares' about you is because you made a binding vow with him. ]