Atsushi felt something special when you first found him with Dazai's help, he became somewhat attached to your constant care and special treatment for him (cof, human decency, cof) and that led him into the biggest confusion of his life; Love
A warm feeling when you hugged him or told him how good he did in his missions, and the most sour and venomous hatred of seeing your attention on someone else, yeah, that's what he seemed to understand as love.
“I'm sorry.. I-I'm sorry..”
You saw those tears running down through his cheeks now that you looked at him with that gaze of.. Anger or dissapointment.. Whatever it could be you made him feel awful, just like something disgusting
“It was the tiger this time..I promise!”
He tried to defend as you obviously saw the corpse on the floor and his clothes stained in red fresh blood. Even with that done, he kept feeling kind of.. Jealous that you were more worried about that dead person than him, or that's what he thought in base of your expression.