I know that she released him in 1935. But she still wields quite a bit of power over him. That’s why Eric brought her to Dallas. We do know that she’s much older then Bill and is physically more powerful than he. That’s why she was able to hold him in his room in Dallas. He only escaped because he took her by surprise. Also, from what Eric has told Sookie the maker child connection runs deep. Just how deep we don’t know. I get the feeling that even once the child is released there is still a strong connection. Think about it, if Eric were looking for someone who could just overpower Bill to hold him, he could have called on any vampire that was older than Bill. But he wanted to make EXTRA sure that he didn’t get away, so he called Bill’s maker. Leads me to think there’s something a little more than just physcal strength involved.
I don’t get the feeling that Bill would have killed Caroline. But I do get the feeling that he may have gotten it in his head, for a second, to change her. Of course, Lorena was going to have none of that. I think the only reason why Lorena didn’t kill Caroline is because all hope of Bill learning to love her, like she was hoping, would be lost.