Being a gifted man of music, song and myriad other talents, Erik had a mental disorder as well as a physical one. Speaking of the original novel, many instances displayed his actions as truly puerile and unpredictable. For instance, in the original novel, the Persian clearly states that he would often remind Erik that he saved his life to keep Erik from killing him. Another time recalls Erik catching the Persian by the lake entrance from the street. Here, the Persian came within an inch of his life until he finally talked the Phantom out of killing him. On the same note, he releases the Persian, sits in the boat and starts belting out a song while kicking his heels on the boat's floor in time with the rhythm.
Possessed by the spirit of the Phantom? No! He was indeed the Phantom, mysterious, gifted, a definite genius, but without a doubt quiet mad.