Something Rotten

Before Mukuro's debut in the series, he was actually still in prison. Many die when trying to corner him. He's a dangerous individual despite his young age. Luckily, he had escaped before his execution.

What timing! Unfortunately, after his encounter with Tsuna and the gang, he is thrown back into his prison for his wrong deeds. He attempts another escape afterwards but fails only to be thrown in the deepest and darkest depths of the Vendice Prison—a water cell to be exact. Strange...Isn't he supposed to be dead? Shouldn't they have scheduled another execution date for this mass murderer? That's where Iemitsu Sawada comes in.

         

Mukuro defending a Mafioso? It seems highly unlikely due to his history, but with the situation he is in, Mukuro has little to no choice with Ken and Chikusa's lives on the line. With this deal between Iemitsu and himself, there is only one problem—Mukuro's inevitable execution. Now that Mukuro has the Vongola's protection for his friends, it now extends to his own life. It was never mentioned in the manga itself, but I think that Iemitsu, using his powers as a high-authority Vongola member, made a deal with the Vendice to keep Mukuro alive so that he may serve his purpose as Tsuna's guardian with the help of his replacement, Chrome Dokuro.