Please accept my condolences on your loss.
Yes, you did the right thing.......elderly kitties with renal failure don't do well. I know from experience that the decision to put down a beloved pet is one of the hardest one ever has to make in this life, but when made out of love and concern for relieving the animal's misery, it is also the most loving thing you can do for them.
If you can, please take some time off. What you are going through is a very normal grief process, and you need time to mourn your pet and accept his loss. Don't let anyone tell you he was 'just' a cat.......he was a member of your family.
My heart goes out to you.