Try not to beat yourself up about it. It was a freak accident, and things like that happen. We have all had horrible things happen to our fishy friends, myself included, and the way I see it the best thing you can do for him now is try to prevent it from happening again as best as possible and caring for your other inhabitants the best you can. I work at a pet store and last year we had a small 8-10in jeweled moray eel. I was feeding everyone one day and didnt realize that the eel had gotten out of the tank and was on the floor. I was looking up at the higher tanks and stepped right on top of his head. I felt all kinds of profoundly disturbing crunching underneath my shoe and I nearly had a panic attack when I realized it was the eel. In the end he healed up fine after staying in seclusion for several days but I made certain to spoil that animal as much as an eel can be. Moral of the story: as much as we love our animals, sometimes stuff happens despite our best care, but all we can do is move on and add everything to our knowledge/skills.
Wow that ended up a lot longer than I intended!