From the well-translated version.
Those who died while cursing will be turned into a devil. These beings come at night and brought death with them. Only the magical singing from a magical boy can kill such devils. Which is why the adult tried to gather those children and forced them to sing to protect Themselves.
When the devil showed up, the devil said it will grant the boy 1 last wish before killing him. The boy wish is for the devil to listen to his song. It turned out this boy’s singing is so terrible. Terrible to the point that the devil decided to teach him how to sing correctly.
In day, the boy stay alone to practice singing. At night, the devil will come and listen to his song. Thing just carry out like that.
The fact is the boy know how to sing, he just don’t want to because his singing will kill the devil. He wants to use the excuse that he is terrible at singing to bond with the devil.