You’d think the start was when the body was found. That’s the beginning of the crime investigation. It isn’t, of course, the crime’s beginnings. For that, you need to slip into a wayback machine and ride time to when the killer was young and beginning their career, back to before the victim and killer had ever met, back to a nascent moment when everyone was happy and oblivious to the future.
After all, the killer just wanted revenge. Their victim had killed first, but the body hadn’t been found. At least, that’s what the killer believed.
They were always one to act on their beliefs.