For anyone curious to know, I have more information on his passing. Took his sister to the vet and described how he had died to her. (Avoiding the disturbing details of that, in case it may be triggering.) The vet told me what condition it was, that it's untreatable and unpreventable, and that there was nothing that I could have done. He died quickly with minimal pain.
While I still grieve for him, I have a little more peace of mind knowing that it wasn't my fault. That it just happened, and that's how it is.