He wants to punish Tulsi killer,that means revenge for her death. His eyes were mix of pain and desire to get to the truth.
Not so out of nowhere,if Gungun heard when D told mr X she was behind Tulsi death. Did not show Gungun reaction then but was sure present and impossible to not hear. Plus when R was blamed had the chance to investigate and confirm it was D. Took a break cause other tracks were introduced.
Great finally adress the ghost in the room and Mohan is not made to trust D,no ml butchered for villain sake. It will be a game and probably D will know sooner or later he tries to fool/trap her,always does. But at least topic is open again and can reveal what really happened that day. D or anyone else as killer.