Who else could have done it?

Which is why i think the pair of them make such a fascinating dynamic, romantically or not. Light would absolutely meet his match with Naomi. She's older than him, she's smarter than almost everyone around him. Light likes to be challenged, and oh boy can she can challenge him. she can keep up with him, and i think this would make him act WORSE. Light is used to being the smartest person in the room. he's used to controlling his relationships with women. now here's this woman who is not only matching him intellectually but is also older and more experienced than he is. i think he'd be simultaneously insulted by it and fascinated by it, because he cant figure her out. And on Naomi's side i think she'd genuinely enjoy having power over him. after being in a relationship with Raye, who was constantly telling her not to get involved and not to investigate and not to do this and not to do that, i think part of her would love having a relationship where SHE gets to call the shots. In an au where she somehow suspects something about Light's involvement in her fiancés death, it would become extra complicated because the only reason she feels justified treating Light that way is because he killed Raye. so now you've got this awful situation where she's grieving, angry, traumatized, and trying to maintain control over the man who ruined her life while simultaneously becoming emotionally dependent on him.

Another layer that could be added to make everything even more toxic is that i also think she'd be jealous of Light's proximity to L. you're telling me she spent years becoming an investigator, years building a reputation and proving herself, and somehow this random teenager gets to sit next to L and work directly on the Kira case? Yeah I would be jealous too. and meanwhile Light would absolutely get weird whenever Naomi talks about how much she respects L because light cannot handle not being the smartest or most admired person in the room. They're both approval-seeking people, too, which is part of why they work. Naomi needs validation because she doesn't feel exceptional, while Light craves praise because he believes he is exceptional, and smarter then everyone else. I have a feeling they would feed into each other's worst habits constantly.

Also, i think Naomi and light's ideologies might overlap in ways neither of them would be comfortable admitting. Obviously Naomi would oppose Kira, even before Raye dies. she values justice too much not to. but she's also not some naive idealist. in LABB she literally says some people are better off dead. i always wonder what her first reaction to Kira was before she learned more. Perhaps she would think "i understand why people would think this is good."