That's so sad. I'm so sorry things worked out that way, but you did make the right choice.
I'm actually experiencing the same sort of thing with my dog, except maybe not to the same degree, and we did adopted her from a reputable SPCA shelter...
My dog, Sabina, was an abuse case. She was unsocialized, beaten, starved, etc... At the shelter she was a doll - very playful, a bit timid at times, but respectful, and just seemed to have the personality I was looking for. After a few visits and "playtimes" I decided to officially make her mine and brought her home. Now, lucky for me she is great with my cat and chickens but other dogs and 'strange' people (mostly 'strange' men) are her targets (and of course she loves to hunt any wild critter that happens to trek through our yard).
I've been working very hard with Sabina to get her over some of her issues and to try to properly socialize her (although it may be too late for her since she missed that milestone). I have been doing a lot of basic obedience training with her and she has made leaps of progress. She has become very submissive to me and my family, she getting better with visitors, complete strangers are still iffy but better aggression-wise; however, she still sometimes fear-bites people, they move too quickly and she snaps at them and other dogs are a NO! no matter how much I try to get her to relax around them (other adult dogs, she LOVES puppies). I also can relate to what you said about the 'focus thing.' Sabina's prey drive is so strong that once she locks on to something, it's like she can't hear me...she almost goes into a trance. I have to literally scream at her to break that concentration and reclaim her focus.... it gets so frustrating at times.
I really think you made the right choice with him... lucky (but sadly for me) I do not have another dog, nor do I have anything that she considers prey, but if any of my family, friends, or other pets were at risk because of her; she'd be gone in the blink of an eye. As someone mentioned, not all dogs can be saved or are worth saving [over your loved ones].
I hope this experience does not deter you from getting a Rott in the future...they really are great dogs in general.