When an animal has become so ill that life becomes a burden, the kindest thing we can do for them is to let them go. You absolutely did the right thing. It was an act of love and mercy, even if you were manic at the time. I think that is what you regret the most, the fact that you were in a mood episode when you made that decision. Please be gentle with yourself.....I'm sure your kitty understood in his own way and was happy to be released from pain and suffering. (((HUGS)))