I think a core part of Light’s character is that he has to be an unsympathetic case study on hubris. Having him be a beta faking alpha status fulfills two key tenants of his character:
- He throws away a decent future for the slim chance he’ll shine bright enough to blind people.
- His only reason is the desire to be the most special.
It’s canon that Light could have a great life if he just took advantage of what was in front of him. Instead, he throws it away in pursuit of privilege and absolute power. He is intelligent, socially adept, and the son of a high ranking cop. He has every reasonable advantage afforded to an upper middle class man in Japan. Being a beta would be perfectly fine. He’d be seen as a steady, reliable person, but not extra special. For that, he’d have to be an alpha on top of everything else.
Now let’s address the other option. Light as an omega faking alpha status is an interesting spin, but I believe it only satisfies half the requirements for his character. In the end, he still goes off the deep end. As an omega though, there’s an argument that he didn’t jump, he was pushed. Like, we’d all be on the verge of becoming the fucking Joker (*waves in trans dude*) after facing discriminatory bullshit. That would fundamentally alter the narrative. Instead of, “Privileged cop’s kid throws away promising future to become mass murderer in pursuit of godhood,” it would be, “Underdog minority lashes out after having to hide status to survive.”
The second option is a good story. It’s also not Light Yagami’s narrative as he is portrayed in canon. This is not me saying don’t write it—holy shit, tag me, I fucking love those takes—just that I firmly believe that canon Light would be a beta who faked alpha status for social status.
TL;DR: He’s gotta be a beta faking alpha status because Light is a goddamn pickme. He will never be happy with a guaranteed gold medal. He wants to be the judge so he can declare himself the ultimate winner. Putting the podium on a necklace is what kills him.