Not every shelter animal is adoptable, and not every animal adopted is suitable for the adopter. This is a heart-breaking situation -- my hubby and I adopted Ben the cat, only to have him become increasingly aggressive and unpredictable. He attacked my hubby twice in two weeks, and became so aggressive that we had to isolate him in a room for our own protection. We thought he would "outgrow" this, but he just got worse. This, coupled with presence of small children in our neighborhood (what if he got out and attacked one of them? Especially since we know of his unpredictable aggressive tendencies???), meant we had to give him back to the shelter. Our local animal shelter said, when we told them of his behavioural issues, that we were the third family to return him for the same reason and that they thought he was "crazy" when they brought him in. But, as there is a disclaimer on their adoption forms that removes all responsibility for the pet's behaviour, there was nothing we could do (not that we wanted the $$$ back, but we wanted to be sure that a family with small children wouldn't be at risk!). We subsequently got Sammy the cat from them, and life couldn't be nicer.
You did the right thing, for you, your community, and your peace of mind.
Enjoy Axel!!!!