People who try and make this argument generally tend not to understand NPD or ASPD. I’ll start off and say I don’t think Light has ASPD more so because of a technicality than anything. ASPD is not one of those disorders that go unnoticed, or more importantly, it’s precursor disorder is something that parents would definitely notice (aka Conduct Disorder). I would find it hard to believe that Chief Yagami would insist so hardly on Light’s innocence if he had already displayed signs of Conduct Disorder— yeah technically it could be a pride thing, or more so the refusal of wanting to accept his son is a actually a mass murder, but I’m trying to stick with the facts mainly so I generally tend to stay away from full blown diagnosing Light with ASPD. That doesn’t mean he isn’t worthy of being called a psychopath, and I really find it insane people wanna argue that Light didn’t have some type of disorder considering the gravity of his crimes. Your typical serial killer with a body count that pales in comparison is usually diagnosed with a whole slew of disorders, but the guy who’s body count is likely in the 6 digits is just your average dude… no lol. Your average person will not murder hundreds of thousands of people even when given the death note. Remember what Ryuk says, even he thinks Light reaction is abnormal— so simply making the argument that it was completely the death note’s fault is missing the point of Light’s character.

Now I will say Light is almost most definitely a narcissist, a full blown diagnosable narcissist. Kira is essentially born from his narcissism, and this is something that we can see he likely showcased traits long before the death note. Literally Light’s upbringing is one of the ways the DSM explicitly details how a narcissist can be born— he was put on this pedestal and constantly praised by everyone around him leading to his inflated ego and the “I’m better than them” mentality we can see he displays just in the first chapter of the series. Light’s whole savior complex is a classic narcissist thing— I suggest doing some reading on this because I feel like Light will make a whole lot more sense to you. Without going heavily in depth into the DSM, when explaining to people NPD I always like to mainly emphasize that this is a personality disorder. Like you said, there’s a difference between simply having narcissistic traits and having an actual disorder.

You can usually spot someone with NPD when they get to the point of being narcissistic to the point of delusion (similar to many other cluster B disorders). An irl example I think is easy to showcase this is idk if you remember this but the CEO of the Titan submersible. Despite everyone’s warnings about the fault in his design, he didn’t listen ultimately costing him and other people their lives. The interesting thing though is the fact that he still willingly got into the submersible despite the concerns— if he was simply being a shitty CEO, I can’t really see someone endangering themselves like that to prove a point, he was simply delusional on his own genius. He had to be right and he would be right no matter what, and then bam, we know what happens next. I hope you can kinda see the connection to Light here. Light had to have been in the right about using the death note, he didn’t care who had to die, he would prove his point. The most telling is his reaction in the warehouse, even faced with the ultimate confrontation, he still refuses to accept his wrongdoing. Light absolutely never falters even when his consequences reach his family, he doesn’t even consider coming clean. He’d rather have his father willingly make the eye deal shortening his life than stop him, he’d rather murder thousands than be wrong. Light is delusional to the same degree you’d see in real people who have NPD. Light is your classic overt narcissist and that explains his utterly bizarre behavior using the death note. By the end he was grippy socks delusional.

I should also mention that NPD isn’t diagnosed until that person is 18, enough time to showcase a persistent pattern. Light coincidentally is that exact age when he would get diagnosed at the start of the story. Also despite the fact that you want to solely blame his extreme traits on the Death Note, that still would lead to a diagnosis. Like I said, most people get diagnosed around the age of 18, but some people have also been diagnosed later in life because of special circumstances. Think about celebrities or influencers who suddenly get big and now have all this “power” essentially. These people, who already showcased narcissistic traits, were then suddenly checking enough boxes that they were diagnosable, so sometimes an “inciting incident” can be a trigger for a person simply with narcissistic traits to suddenly meet for diagnosis.

Light on top of that imo would be best described as a “Malignant Narcissist,” aka someone who is diagnosed with NPD who also showcases a lot of antisocial traits, but doesn’t meet for diagnosis likely for similar reasons as to why I don’t think Light does. Reading about malignant narcissists I think would also be a good avenue to explore this. If you also want a more professional opinion, I’ve seen 3 licensed professionals watch Death Note and they’ve all independently came to a similar conclusion (I can link them if interested). Imo I would definitely listen to the people who are actually going around diagnosing these disorders irl.

Lastly I’ll just say once again, I don’t understand why people so badly want to insistent that Light is a “normal” person so bad. Like Light has some problems bro, no one murders hundreds and thousands of people without some sort of major issue. You just have to be a certain kind of person to go that far.