Just my two cents...I know you must be miserable right now, and it's understandable that you don't want someone who threatened your life around your daughter. But re: the letters, I think it sounds like you know from experience no matter how bad a parent's made out to be, a child still has an innate desire to know them and will find a way. If you try to cut off all contact, your daughter could resent that when she's older, no matter how illogical. Again, just an idea, but I think you should save the letters in some forgotten corner or even give them to your lawyer to hold and them when she's 18, or younger if she's mature. You can then sit her down, explain why she doesn't have contact with her biological father and that he was a threat to her safety, and then show her the letters and say you trust her judgement to do what them as she sees fit. That's assuming, of course, the letters are appropriate. It seems you regret not believing your dad about your mom, but when a kid's told not to do something they're gonna do it. If you treat her like an adult and let her make her own judgement, it's much more likely that she'll trust your opinion of him and not think you're trying to hide something, run off and get in a position like you had to deal with your mom. Might be the best way to protect her.
Again, just my two cents after stumbling accross this thread. Hope I didn't say anything wrong! Best of luck.