Ainsworth controlled the Blood Puppets to transmit the virus to those assassins. After he assimilated them into his Hive Mind, he accessed their memories, and they had indeed been hired by Caspian, who had used his father's money to hire them.
With both father and son being involved in his assassination, Ainsworth no longer had any scruples about slaughtering the Sterling family.
"I was planning to kill these assassins, but to give Caspian a taste of his own medicine, I might as well pass on the demon race's torture techniques to those assassins and let them torture Caspian."
With that plan in mind, Ainsworth used the Hive Mind to transfer memories to those assassins.
He had already released the Forbidden Magic: Blood Sacrifice - Cage spell, so all the assassins were no longer imprisoned by him.
