He had also uncovered the rather shocking mystery behind his existence which was basically to stop a feared and powerful villain from destroying the who'll world in exchange for his life.

Thinking about it, Asura found it funny! Indeed the heavens were unfair! all these While they'd watched him train and go through hell just to exchange his life for this? it was funny yet amusing...

It was probably why he laughed it through.

And at the same time, he couldn't help but look at the good side of it all which was the fact that he had taken his revenge!

The two basic objects of his revenge who were The Heavenly emperor and ZhanShen were no more. And that was an achievement. it was at least something to him 

And he could say that although he couldn't go deep into destroying the heavens, although his life he had managed to prove himself to the three realms and tell them his presence