Silent Reasoning

Young pianist Chen Qibai is accused of murdering his mentor, piano education guru Song Huaiqing, in a secret room. The murder weapon was covered with his fingerprints, and no one could prove his absence at the time of the crime. The 300,000 usury loan and an unfair performance contract provided him with sufficient motives for the murder. Chen Qibai's wife Han Shumin was desperate and found Qin Mo. She insisted that her husband was not guilty, and brought a key piece of information - on the night of the crime, Chen Qibai finally completed the "Fourth Movement" of the sonata he had composed for more than a year, and excitedly said that he would "play it for Teacher Song." What reason does a person who has just completed his hard work have to kill his mentor who most wanted to show his work that night?