What is the ruling on a doctor working in a prison, and would he be sinful if he provided healthcare to criminals?

Imprisonment does not negate the rights of a prisoner, including healthcare. Leaving a prisoner without treatment leads to their demise, which is impermissible. If there is no one to treat him in prison, he is released for this purpose, and once treated, he is returned to prison. Muslims have historically cared for sick prisoners since ancient times, and caliphs allocated budgets and physicians for their healthcare.