Seikou's mask was lying broken and shattered and fragmented on the ground around him. For some reason, this didn't bother him yet. He just felt cold, perhaps mildly annoyed.
Then he realized why. They had always said that it was his soul. His soul was broken. His soul was in itsy bitsy pieces on the ground.
He picked up a fragment of the mask - a fragment of his soul, really. And then it struck him as incredibly amusing that he could pick up his own soul, that souls could break, that something that eternal was also so fragile...
And he began to laugh.
He laughed at how silly it was, at how stupid the causes of this were, at how incredibly absurd it was that souls could break. He laughed at the masks, at the world, at himself. He laughed until it hurt to make a sound anymore, until he could barely even breathe, until there was nothing left to laugh at. And he did not stop.
He had forgotten why he was laughing, but he knew inside.
If he stopped laughing, he'd have to face those shattered pieces again.
And he couldn't do that, so instead, he just continued to laugh himself to death.