I can remember this one so vividly, but not the title or enough exact phrases in order to google it!
Still in their youth, Thor and Loki are summoned together before Odin's court. Loki is commanded to wear white, and wonders if he is going to be betrothed in an alliance marriage. Odin, however, strips Loki of all concealment illusions and denounces him as a monstrous beast. Thor is commanded to kill Loki. He does.
Thor grows to realize he must rule by conquest and fear, and proceeds to take over Asgard. Anyone powerful could be a threat, and he eliminates them step by step. Frigga is locked into a luxurious tower filled with entertainment material and generous food reserves that, with careful allotment, could last her centuries. Odin is locked in a different tower. His food reserves are not nearly as generous.
The story ended with Thor sitting on his throne, talking to Sif and the Warriors Three. But, friends could also be considered threats, and he'd had them killed ages go and mounted their heads in the throne room.
Any ideas? I think I may have read on Fanfiction.net circa 2011. THANKS!