Oh, how sad! Hopefully Kitty will find an owner who will be able to figure out what her litterbox issues were. She may just want to be an "only cat" which of course was not possible at your house.

I would not add another cat at this point. How long had it been since Kitty went to the Humane Society? Is do Cookie and Baby Kitty get along? Is Cookie eating and pooping normally? How old is he? From the sound of it, there may be something medical going on - pain will often cause aggression. A vet trip is in order, and then if you get the all clear on his health, we can help you with him emotionally!