Demons
Thor
Thor's demons are people he never thought of, those he has always taken for granted. He wonders to himself, on dark nights when Jane's busy, when the team is occupied, when they've finished another mission, how many he has killed with no meaning. The events of Jotunheim haunt him, chase him, remind him of foolish mistakes, of innocents lost because of his own bravado. He wonders.
Once he asked his father about the number of deaths he has caused, asks if they were all deserved. Odin had never really answered, instead drawing himself up, looking Thor in the eye, and saying, "That is who you were, Thor. The mistakes of the past no longer matter."
It plagued Thor. It was confirmation. He had killed needlessly, and no one cared, because he was better than that now. But it was because he was better that he cared.
Loki, too, became an obsession for the demigod. His brother's face crossed his mind often, the grin of joy, of triumph, of innocence. But with the smile came the tears, the screams, the ravings that Loki had just wanted to be like his big brother, to be a hero, a king, not a monster.
And now Frigga, his dead mother, takes her place among the ghosts, and she is the worst of them all. Frigga's death could have, should have, been prevented. But he had traded the life of his mother for Jane, and he can never fix it.
Tony
Tony's demons are nothing like Thor's. Tony's demons are actually limited to one, but that one haunts him as Loki chases Thor. Tony hates himself. He hates the staggering, drunken, smirking, callous mess he used to be. He hates looking at Pepper and seeing her smile falter as he relives his past, hates the way Steve eyes him, waiting for him to make another quip at his uselessness, hates the way he knows Natasha and Clint see him as a bomb, one that they can't know is disabled.
He hates the memories. He hates how he can remember every insult he has ever given to his teammates, all the pain he has caused Pepper.
But Tony is not like Thor. Tony knows he is he is not that Tony anymore. So every day he sets out, pushing the old Tony further and further behind him, beating his demons back under the bed with an AK47. And he's fine with that, because someday, someday, that demon will be dead, buried, and forgotten.
Bruce
Bruce's demon is the opposite of Tony's. Bruce's demon is who he could have been.
Some nights the scientist stares at the ceiling, alone in his king sized four poster, and thinks. He remembers Betty, how she wanted four children, two boys and two girls. He remembers how they planned, how he had been planning to propose, how nervous he had been when he bought her the diamond ring she had longed for. He didn't know where the ring was, only that it was identical to the one Tony had picked out for Pepper.
But still, he dreams. He sees himself coming home from the lab, a small girl with his dark curls and Betty's eyes running to hug him, his sons wrestling on the kitchen floor, and Betty pushing a toddler into his arms as she kisses him. He sees burnt suppers as the two of them figure out that just because they're scientists, they're not really very good cooks. He sees christmases and birthdays, soccer games and ballet recitals. Most of all, he sees himself there, and he sees Betty beside him, holding his hand, and they're smiling at each other.
But then he shakes himself, curls into a tight ball, and pulls the blanket around him. He will never know that Bruce Banner.
Clint
Clint's demon is the voice inside his head. Soft, reassuring, convincing, Loki orders him to kill Natasha, to kill them all. Even long after the god's influence has been broken, after he was taken to Asgard and punished, Clint still hears him, telling him to kill innocents.
After particularly long nights, he'll sit at the breakfast table, picking at the bacon and waffles Steve made, and avoid his team's gazes, wondering if they can even guess how much he has to fight the quiet voice, telling him to stab Natasha, break Tony's neck, shoot Thor. He thinks Steve knows. Steve knows everything about his team, from Natasha's favorite tea to Bruce's pet peeve (wet socks) to Thor's preoccupation with junk food. But Steve doesn't ask about it, just pats his shoulder and refills his coffee mug.
Natasha
Natasha's fear is her ledger. It doesn't drip, but it gushes, just like Loki said. She counts the kills she has made in her head, and finds that the number is a blur. It's the past, she repeats, she's better than that now. She's an Avenger, and she kicks ass and saves lives alongside Captain America and the god of thunder.
But the red splashes the ground every time she opens the mental manilla envelopes, and she has to close them up to avoid drowning.
Red stains her fingers, and desperation clouds her heart as she blots at the ledger with a paper towel.
Steve
Steve faces the worst demons. They never relent, never pull away, never stop. A smile from Natasha sends him reeling back as Peggy thrusts herself into his mind. Clint throws fries at Tony, and suddenly Bucky and Howard are verbally sparring. Bruce chuckles, and DumDum adjusts his hat. Thor combs his hair and plays video games, and Charles is fiddling with his newest gadget.
He can't escape. He wakes up every morning, and faces a team that is too similar and too different from the Howling Commandos. The banter plagues him, Tony fiddling with his phone, his tongue sticking out (An old habit of Howard's) slams him in the stomach every morning as he tries desperately to adjust.
As he tries to understand the TV and five remotes that go with it, he sees Bucky tossing a pillow at his head and telling him to get on with it. As he slams punching bags, he hears DumDum coming up behind him and offering a cup of coffee, but when Steve turns around with a smile on his face, it's just Sam. And the smile fades.
But worst of all are Bucky and Peggy, always staring at him, everywhere he goes. In a team as closed off as the Avengers, Steve misses the open and ever present companionship that the war offered. And he knows, even as he grins at Thor and sets a casserole on the table, that the demons will never go away.