What caused my cat to die when I found him lying dead with a pool of blood by his nose?

It is not possible for me to provide a specific diagnosis of what caused your cat to die based solely on the information you have provided. There are many potential causes of death in cats, including trauma, poisoning, infectious diseases, and underlying medical conditions. Without a thorough examination and diagnostic testing, it is impossible to say definitively what led to your cat's death. If you are concerned about your cat's death, it is important to talk to your veterinarian who can provide a thorough evaluation and advice on any further steps that may be necessary.