CM -
As others have already said, don't blame yourself for this. It sounds like you tried to steer them towards being responsible owners, and they chose to be otherwise.
My family is in the same situation. . .one of my aunts had a BC mix that bit a couple of people (including a child). She gave it to my grandparents, where it bit the mail carrier twice. It's still alive. The excuse is that "it's a herding breed. . .they're supposed to bite."
Then, my other aunt has a half-blind, intact male Yorkie. The little dog is carbon-based organism-aggressive. I'm serious. . .it is a nasty little dog. It's "okay," though -- it's just a little dog (note sarcasm).
Don't be too hard on yourself. . .you did what you could. :(
Audrey