Bichon Frise Rescue/Adoption

I adopted a rescue Bichon Frise from a local animal shelter in the hopes of finding a good pet. He was so lonely looking and timid, my heart went out to him and I took him home with me that very day.
It didn't take long for me to realize that my sweet rescue dog was afraid of nearly everything. He seemed to be untrained and when he was scared, he became aggressive.
After two years of rigorous training, I still couldn't let him off lead, he couldn't be trusted with children, men or other dogs. In the end, he did bite someone and I had to have him put to sleep.
I know that not all rescue Bichon Frises are going to be like the one that I chose to try and help. The situation definitely calls for more details concerning the Bichon Frises past and some deep consideration.