Arvin was not a clean freak. But there was no way in hell that he wanted to keep a dead person's body in his inventory. So he had already decided that he would dump the body of his disguise on the same bed as Aria. He would stick around. He would surely like to see Aria's reaction when she would wake up tomorrow.
Imagine waking up with a dead body beside you. And that body was brutally murdered. The neck of the dead body is broken while some of the ribs of the body may be fractured. It would nice scene to watch. So after dumping the body of the cat-man that he killed, Arvin transformed into a fly and flew away. He would like to explore this mansion.
