@furopoi
Nope mate. It depends on the criminal law in country but normaly if the subject of the crime does not exist then you can't treat it as commiting that crime. Let me use an easy example:
You can't steal your own money
You cannot be sentenced for battery/assault if the only thing you did was hitting sandbag with a picture of your most hated person. Its not really that person.
So you cannot be charged with killing person who already died. Like the MC said it can be a diffrent crime of desecrating corpse or simmilar.
Plus this is just a manga that omits many of important parts of law. Treat it the same way as you treat those silly games of Phoenix Wright (ace attorney). Its just entertainment. It would be too complicated otherwise and not as flashy.