Why does Raskolnikov keep laughing? Why the despair?

His laughter comes from nervousness..... the despair is found in his guilt and fear of being found out.

“But when has such a thing happened? I never have anything to do with the police! And why just to-day?” he thought in agonising bewilderment. “Good God, only get it over soon!”
He was flinging himself on his knees to pray, but broke into laughter—not at the idea of prayer, but at himself.
He began, hurriedly dressing. “If I’m lost, I am lost, I don’t care! Shall I put the sock on?” he suddenly wondered, “it will get dustier still and the traces will be gone.”
But no sooner had he put it on than he pulled it off again in loathing and horror. He pulled it off, but reflecting that he had no other socks, he picked it up and put it on again—and again he laughed.
“That’s all conventional, that’s all relative, merely a way of looking at it,” he thought in a flash, but only on the top surface of his mind, while he was shuddering all over, “there, I’ve got it on! I have finished by getting it on!”
But his laughter was quickly followed by despair.