Read the novel's forum because I've been seeing that ML was supposedly a red flag but here's what I got:
ML being a tutor was a big elaborate plan by MC's mother to look onto his son's school life. She was worried that he's being bullied so she hired ML (who was kinda close with his son's age) to be her informant. ML agreed because he was orphaned and the mother was a sponsor, so in gratitude he agreed with the plan. From what i got, ML did really "manipulate" MC into opening himself to ML but his actions were still of course genuine, like he was also worried abt the bullying case. Like he went there to do a job but he also felt sorry . He wanted to confess abt how he git his tutoring roles but he was scared (?) to admit.
Though he didn't also overstep things--he didn't became the knight that saved MC from bullies. He just relayed the news as per his job's prescription. I think this is where people are torn between him being good or not. Anyways, stuff happened and MC went overseas.
Years later, they met and ML was able to recognize MC after hearing his name. He wants to confess and talk about the tutoring issue but he further contemplated when MC acted that he didn't ML. Long story short, he does plan to spill the beans but MC's mother beat him to it, further straining the advancement of their growing relationship.
My conclusion is, people are overusing the term red flag for people's mistakes when those mishaps can be overturned by their good deeds. Yes, it was wrong for him to be "spy" but remember that ML was just doing it out of gratitude bc Mc's mom was the sponsor of his orphanage. Second, from his POV, he just saw a mother who was worried about her son's not opening up to her about his problems. It came from zero malice. He did plan about spilling the beans during their past but he didn't had the guts to do so. Calling him red flag because of those is a stretch. He had mistakes but no one's perfect