I still think Zagi-san may have been the one to do the mess in the Library. He wouldn't have been afraid of inconveniencing the Kingmaker.

The culprit must have caught him in that moment and the fight ensued in the Dining Hall. THe Library and the Dining Hall are pretty close, after all.

Then after the fight the culprit returned to their room, bleeding all the way and...took a chair with them...? There was a chair in the hallway...I think?

[That chair confuses Mouri. Yay]