Not really adhering to the prompt. Title from Nabokov's Lolita.
Disclaimer: Thor is not mine.
THIS IS DARK, I"M WARNING YOU!
Frigga is not a bad mother.
She succesfully raises Thor as the epitome of a true Asgardian. She takes Loki into her family, always treating him like he is her own. Other mothers come to her when they have run out of ideas of how to deal with their children.
Loki fascinates Frigga the way no man has ever before.
Ever since Loki hits puberty, Frigga can't help but seeing Loki from a different perspective. His dark hair, his pale skin, his slight body...Frigga often fantasizes about how Loki would look coming undone beneath her. How Loki's face would flush, his mouth open, gasping and crying her name...
Delicious, Frigga thinks when she looks at her second son.
She begins to spend more and more time with Loki, together in what she tells her husband some "mother-son quality time". She touches Loki more often now, holding his hand when they stroll through the garden, hugging him like he's still a child, kissing him goodnight.
She can see the confusion in Loki's eyes. Frigga does not doubt that Loki suspect her behaviour as unmotherly. Even though Loki has no sexual experience, she knows that Loki is not so naive in such matters. There are books about that in the library, after all. Knowing Loki, he probably has read most of them.
Frigga devises a plan.
It's the perfect time. Odin is away for a diplomatic visit, Thor's with his friends hunting. No one will suspect, and more importantly, no one will ruin her plan.
That night, Frigga brings Loki to her room. When she locks the door, something like fear crosses Loki's eyes, but she kisses it away.
Finally, they are fully bared, and soon, gasps and moans filled the room. It's even better than Frigga's fantasies, the way Loki's virginal body yields to her caresses.
It is just the beginning of many more nights.
Frigga is not a bad mother. If Odin accuse her of such depravity, well, Loki is not really her son, is he?