Grief and guilt interconnected. With every death, human or animal, we have guilt, the could of, would of, should of. We add to this that sometimes we were annoyed by them, sometimes we maybe said horrible things, which we beat ourselves with. The thing with grief is that we have to rationalise it. Why did they die? How could this happen? The problem with this is that when we try to make sense of it, we often make it seem somehow controllable.
They died because A happened, which led to B happening. If we could have stopped A or B or both from happening, they wouldn't have died and everything would be ok. In our heads preventing A and B from happening suddenly becomes really easy and obvious. We should have known. Not only did we let A and B happen, but we let it happen despite it being really obvious 'at the time' that we should have stopped it.
So not only did we not stop them from dying, but we knowingly didn't act, even though it was blindingly obvious we needed to do something and every other person in the whole world would have. Sometimes this changes into a belief that we more or less killed them by our stupidity, laziness, etc.
All of this is natural. Our brains are trying to understand something that is shocking and unusual to us. But it's doing this at the same time we are in shock, distressed and still having to function more or less. As humans we all know that people and animals die every day in their thousands. Car crashes, cancer, heart attacks, being hit by a car etc aren't unusual events, at least not when they happen to other people. It's really sad, but relatively common. But when it happens to someone we love, it isn't a common occurrence. It's the equivalent of a nuclear bomb going off. It is too distressing to be easily explained. Yes, people or animals die from cancer, but not MY dad, not MY cat.
There's no easy way to stop your brains from turning a tragedy into something we actively could and should have prevented, because it's natural. But it can get out of control very easily. If this happens, I literally tell myself to stop. "I am not doing this." I then try to think of something else. This sometimes is so hard, I have to just think of the chair I'm sitting on, a clock on the wall, even a bus going past. I will sometimes just say 'chair' repeatedly in my head. Anything to break the cycle. I may do this a thousand times a day, but it does help, it stops me be consumed with grief for a few seconds, which grows over time to me instinctively knowing that when the rush of grief and guilt comes, I need to focus on something else.
I also remind myself how sad it is to always feel bad and destructive thoughts when I see a photo or someone mentions someone who's died. They would not want you to feel like this. Someone I loved died far too young. He'd fought so hard and it was so cruel. But I realised that it was really sad for my life to be destroyed too. He had fought for the life, I still had. I try to remember this. I'm not living my life for him, but for me. I'm lucky to have it. We all are.