No, you're not a murderer. Sparky's age is probably playing a part in this. Just keep his water clean and make him as comfortable as possible. That's all you can do. Hang in there.
You've got to remember Sparky is very old. :/ His body may just be shutting down after the stress of his tail ripping. Is he suffering? Can you lower the water in his tank so he can breathe?
*Sighs* I'm afraid I don't think Sparky will pull through. He's having trouble swimming now, and his immune system isn't like it used to be. I won't be able to get AQ salt for about a week.