Boss was gone, and a rival had taken his place. Shiryu's wanton torture of prisoners was naturally reported, and he was sentenced to death with a stay of execution and imprisoned.
As for how many times he had been released and locked up again in between, that was unclear.
Did Shiryu not know the consequences when he did those things?
Of course he knew. But so what? He simply did not want to keep doing it, because he could not control himself.
Later, when Blackbeard wreaked havoc in Impel Down, Shiryu's betrayal had been as smooth and natural as flowing clouds and water. He had not held the slightest attachment to Impel Down.
That was all about Shiryu from the original story. The current Shiryu was still following behind King the Conflagration's corpse and was far from becoming what he would be more than twenty years later. There was plenty of room to reshape him.
Carlos believed that if he wanted to lure Shiryu away, Shiryu had to suffer a setback. King the Conflagration needed to pledge himself to Kaido ahead of time, and Shiryu needed to be dealt a brutal blow by Magellan. Otherwise, Shiryu had reached Impel Down's middle and upper ranks at such a young age; he was surely proud and would not be easy to fool...
What, recruit King the Conflagration?
The idea was indeed tempting, but he could not do it. As Impel Down's current boss, King the Conflagration was already at the Admiral level, or at least close to it. Carlos could not overpower him.
Kaido was more feasible, but Carlos still was not strong enough.
There was no need to mention Magellan. Recruiting him was simply impossible. Leaving aside his personality, just the Venom-Venom Fruit alone made it impossible. Wouldn't Carlos have to