I thought he knew from the beginning. It was pretty much in the first chapters that he said something about a "rotten personality" or "rotten to the core".
he didn’t know about APD. a rotten personality and a mental illness are not the same. LW tells him about it later in the novel.
To be fair, top hides it pretty well. He fakes emotions like other people and can't empathize. He's like the top in rivalry, except worse because no one tries to teach him how to process emotions.
he did all this research to understand Jennie’s mental illness but despite all his stalking, he never found out that ml is a sociopath!?? ughhh.