I dont't think you should take all that on yourself and blame yourself like that, you did what you thought was right, just because you tried to have faith in human logic and tenderness for other life doesn't mean you should blame yourself because stubborn ignorant people let you down.
You gave your neighbours the benefit of the doubt and I really think most people would have done the same, they let YOU AND THE KITTENS DOWN you only tried to do your best and be responsible. It's all very well feeling regret in hindsight but I really think you have nothing to berate yourself about, you simply weren't to know they would end up being...sorry for the hardness...but cruel and stupid!
I think you sound like a really generous and caring person and you did all you could.
I think you sound like a really generous and caring person and you did all you could.