"Don't cry! Show some dignity!" they told him. He couldn't remember a time when he'd been allowed to cry. So instead, he laughed.
When he went to Hogwarts, and was Sorted into the wrong house, failing yet again to be good enough, he laughed. The other students, and the teachers, all stared at him.
When his parents sent scathingly cruel letters to him, and the dull pain rose in his chest, he laughed. A little bit wilder, a little bit fiercer every time. And when laughing didn't help, spreading the pain, did.
Cruelty can be learned, and he'd been taught well. He knew just what to say and how to do the most damage, and when the pain wouldn't go away if he laughed, he used the cruelty. And laughed.
He knew he was lucky to have friends who could rein him in, just as he could rein them in. Not that he did very often. There was an exhilaration in running wild, in doing whatever struck his fancy, or that of his friends. To laugh, loud, free and wild. With someone, at someone, or just because he could. He hated limits.
When he pushed too far, in cruel mockery over what he knew and others didn't, and almost, almost, caused unfixable damage to one of his friends (he didn't care about the other life that could have been ruined; why would he? That boy was no friend, and as capable of cruelty as he was himself), he laughed. Because crying wasn't allowed.
Then he was taken back, allowed in again, allowed to laugh and play and prank with his friends again, and life was as it should be.
Leaving his family hurt even more than being banished from his friends. He had never doubted that his friends would take him back, that they loved him as much as he loved them. But he knew, had always known, his family was as immovable as he was himself. What you said, you did. Promises were kept, even those that ripped out your heart and soul. And his parents both promised that if he left the house, if he left the path they meant for him to walk, he would be dead to them, never welcomed back or even acknowledged again. Ever. But he still left. He would not swear himself to the Dark Arts, though he knew more Dark spells than anyone knew. And he would never swear blind obedience to a Lord. So he left, went to the friend who was more a brother than the brother he had by blood. And he laughed as he told what had happened.
He loved fighting the war, hated that it was needed, but loved the risk, the danger. Until people he knew, cared for, started to hurt, and die. Holding the hand of a lover as she cooled, watching the torn bodies of friends. But still, the thrill of fighting made his blood race and his head go clearer than it had ever been before. No past, no future, just the now. He loved it still. Loved it so much those around him started to wonder about him, about his family, about his skill with the spells that touched on the Dark. Sometimes, he forgot himself, and used what his family had taught him, long ago. He laughed as he fought.
Then his brother disappeared, and he went frantic. The boy he'd loved and protected, argued with, fought with and against. Always there, another slice to the wound that was his family, the wound that would never heal, that he would never allow to heal. After all, it wasn't in his nature to stop loving, even when he despised. No body was ever found, but eventually he found out his brother had been killed by the Lord he'd so willingly sworn himself to. He went to the funeral, stood hidden in the back, and when everyone else had left, he went to the empty grave. He laughed, bitterly, brokenly, wishing his brother had come to him for help.
The news of the prophecy struck them all hard. But there was never anything to think about for him. His friend, one of the last shards of family he had left was at risk, another brother was at risk, and a baby he loved, loved, loved. To promise to keep a Secret was nothing, the knowledge of what he would have to endure to protect it was nothing. To see the group of friends, of family, from school be crushed beyond repair by death, was unendurable. But he made them change, because to act as a decoy, to take the risk, but not be able to give anything up, made more sense to him.
Finding the bodies of his closest friend has his wife, finding the living baby, shattered him. There was nothing left now. One dead, one a traitor, one alienated by misguided suspicion. And as he gave up the child, to what he hoped, had to hope, was a good future, he knew his luck had run out. He knew everyone would think it was him, unless…
So he went hunting rats. But the rat slipped away, and with it, the last shred of hope for vengeance, for exoneration. He knew his skill with the dark spells, his willingness to hurt, to be cruel, would now come back and bite him. And perhaps he deserved it? So when they came, when they accused him, he laughed.