I wanted to say again, I can relate a whole lot to what you have shared. I even told my therapist at the time that I knew I was a sociopath. He disagreed with me, very strongly, but I felt like if he had all the details he would agree I was a sociopath. It took a long while and some processing, and I am nowhere near done yet. But I know I am not a sociopath, because I *do* care about animals and definitely *did not* want the bad things to happen.
I wish I had done more to help. I wish I had been able to function enough and in the best (or at least sufficient) way to be a better pet owner. But I can't go back, and I really really need to grieve that loss. I need to grieve it fully. And that scares me. I do intend to join a grief group, you might want to Google "pet grief groups" in your area. I know there are some where I live. I also want to order the book Goodbye, Friend, but even just typing that title makes me cry right now so I don't think I will be able to read it yet. There are a few books like that, about losing an animal. I think it's Goodbye, Friend or a similar book that has a chapter about accidental deaths or deaths where the owner "could have" done something but didn't/ was unable to. I think this would really help you.
Self-forgiveness is very hard to get to, but I think you can take some steps to get there. Finding a grief group, talking to a grief counselor, and reading pet loss focused books should help ease at least a bit of your pain. I intend to take those same steps. I will let you know how it goes. I do think healing is possible.