Maybe it was an accident. Harry Potter was experimenting with some dark magic (which he may have been forced into or just out of curiosity) and things got out of hand, resulting in the creation of the plague.
It could also be a form of revenge. Suppose he felt wronged by the wizarding society for some reason, like being falsely accused or seeing his friends suffer unjustly. Creating a plague might be his extreme way of getting back at them.
Chapter 201, "Passing the Door Three Times without Entering", is the clearest early answer because it pushes Miss Jiang into a new phase or setting. The chapter gives readers a real answer by opening on The third time she passed by happened to be when she was rushing back...
Chapter 253, "She Hit My Head Three Times, I Hit Hers Seven Times", is the clearest early answer because it turns Round Champion's situation into a concrete plot event.
Chapter 349, "Everyone Who Went Out to Dinner that Night Got Poisoned!", is the clearest early answer because it turns Director Zhang's situation into a concrete plot event. A useful checkpoint is the way the scene starts with The news quickly reached Ye Chu.
Chapter 745, "I'm Pregnant", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Several people finally met Shu Yan in the hospital ward.
Chapter 628, "It's Time for Them to Show Themselves!", is the clearest early answer because it turns Time's situation into a concrete plot event. The chapter turns that into something concrete by moving from What are you fantasizing about?" Bai Lu looked at him incredulously,...
Chapter 428, "Stunning Debut!", is the clearest early answer because it turns Xi Yang's situation into a concrete plot event. Early on, I'm going to perform a cover of the song 'The Ocean'..." Jiang Shixu.... By the end, Your voice is truly stunning," Xiao Yi was the first to pick up the....
Chapter 712, "This is definitely not reliable!", is the clearest early answer because it turns Miss Jiang's situation into a concrete plot event.
Chapter 387, "Hello everyone, this is my disciple", is the clearest early answer because a shaky connection becomes usable in the plot. The chapter turns that into something concrete by moving from Liu Suqing V: "Even if we meet, I wouldn't recognize, faces covered...
Chapter 708, "Stinky Beggar", is the clearest early answer because it turns Wen Jiaojiao's situation into a concrete plot event. The chapter gives readers a real answer by opening on Chen'an's executives feared that the situation would escalate and...
Chapter 401, ""Youth Trainee" Premieres!", is the clearest early answer because it turns Huo Xuan's situation into a concrete plot event. The chapter gives readers a real answer by opening on With Huo Xuan's help, Jiang Li quickly got rid of the fans and was...