I'd go with Ham's acid bath thing.
Wait, no. That would be too merciful. They'd still die, whereas their victims would have to suffer the effects of AIDS.
They should be infected with AIDS like their victims were. Then sent to a particularly brutal jail made specifically for scum like them where they'd be beaten up and raped every day. Oh, and have their genitalia cut off. With no anaesthesia.
Wait, no. That would be too merciful. They'd still die, whereas their victims would have to suffer the effects of AIDS.
They should be infected with AIDS like their victims were. Then sent to a particularly brutal jail made specifically for scum like them where they'd be beaten up and raped every day. Oh, and have their genitalia cut off. With no anaesthesia.