They all ask him "why?" Why did he try to destroy the Frost Giants? Why did he try to kill Thor? Why did he attack Midgard? Why does he not repent? Why is he such a monster? Why, why, why? He likes to smile at them when they ask these questions-a wide, toothy grin that he knows makes him look terrifyingly insane. Another "why": why does he smile at their questions? Because he doesn't know the answers. His smile is a lie, like a lock on a chest filled with nothing. People see it, and think that there must be something valuable hidden inside, else why would anyone bother to lock it in the first place?
But there is no key to his smile. There is no lock pick in all the world as can reveal the truth behind it. Until the lock rusts with time and wear, and the wood molders and rots away, his emptiness can remain a secret, and he can pretend that he knows "why."
An empty chest... sometimes he wonders how accurate that comparison is. Surely he is empty of answers, but is his heart also empty? His very being? He has wrapped up his true self in so many dark, twisted lies, he wonders if, underneath, it has withered for lack of light. If he were to heed his brother (smile to hide how much it hurts) and remove the lies from himself-unwrap and discard them one by one-he doesn't know what he'll find.
He's not scared (smile to hide how terrified you are), but he... speculates sometimes that the void is as much inside of him as it was outside (when the demons won't go away, when the lights seem too dark and the air too thin and your skin too cold and it feels like falling and the voices scream too loudly: "why, why, why!?")
He digs his nails into his skin and smiles. The questions stop. No doubt whoever was asking them has turned and fled, but he doesn't know for sure. He doesn't look up, because he never sees them anyway, he only hears their voices resound throughout the little room, buried a thousand miles beneath Asgard's crust. Some form of magic meant to torment him transports the voices to him, he's sure. He refuses to acknowledge that his prison is magic-proof, with only Heimdall's keen eyes able to penetrate it's walls. He turns his mind from the idea that these voices, this guilt, this torment, are his own. He lies to himself, because underneath the lies, he is nothing at all.
A thousand miles above, a weary king makes the long trek up to the Bifrost, where the loyal gatekeeper stands watch over the newly repaired bridge. The king inquires after his adopted son, in a voice that already knows the answer, but weakly dares to hope that it shall be wrong. The dark-skinned man turns his golden eyes to the king-cold eyes that dash even that small spark of hope-and says, as he always says, "He smiles."
For smiles hide so many things.