A very good question.
With a horridly depressing answer.
He'd laugh at first true, bask in the pain as you say, but the depth of Kyusai's character goes deeper than that. There's a massive part of him locked away that hates everything he does, that despises the act of killing. However his life as the abused child of two gangsters who wanted a killer for a son twisted him to the point of almost losing that part of himself.
In your scenario, once he calmed down from the laughter and actually realized that he was finally the one dying... he'd probably use his last breath to thank the person who killed him, sincerely and not sarcastically. He'd finally be free of the monster that he had become, the monster he is incapable of stopping.
Comparing this to a video game or visual novel (as is fitting literally and metaphorically for this RP), then let's say that this outcome is the "Bad End" for Kyusai. Kyusai is completely unable to redeem himself in this result and dies as a villain and nothing more, the only remaining shred of dignity he has fading away in his last words , perhaps leaving whomever dealt the final blow to wonder just who Kyusai was and whom he could have been if given the chance.
Kyusai is more than just your stereotypical psychotic bad guy. Deep down at his core he's just as much a victim of his own actions as all of the people he hurts and manipulates are.
Don't get too down in the dumps yet however, there are "Good Endings" to Kyusai's game as well that I've been going through in my head. There may be ways for Kyusai to finally redeem his actions and break free of what he is, depending on how he interacts with all of your characters in the stories to come.
But... well, I won't go over any of those happier outcomes right now. There has to be some surprise to him after all, right? ^-^
