For everyone having trouble with this contract, where you kill him and the system does not update that he is dead so you can continue the Dark Brotherhood story line:

I didn't get the glitch, but my husband did and this is what worked for him:

? Have a black soul gem in your inventory (if you have purchased the house in Windhelm, there should be 2 there)
? Save inside your house in Windhelm
? Fast Travel to Angas Mill
? Cast Soul Trap on Ennodius Papius or have a bow of Malediction (which traps souls) He must be killed with a bow and arrow.
? Kill Him

It worked for my husband, so make sure you try, maybe it will work for you.

(He had already tried everything else; kill with different weapon, looted his body, etc... )