If anything else then your hero's attack deals the final blow on a hero you do NOT get the +5 damage bonus from death pact.

Things that I tested so far: attacks by Burning Army, Skeleton Walk archers (Aghanim's Scepter), Orchids Soul Burn, Bloodthornes Soul Rend, Dagon, Radiance, Eblade.

This might be intended but that would make death pact rather underwhelming. Burning Army and Archers are a problem imho. I could live with items not working as they are another source of damage and the tooltip says: "If YOU kill an enemy hero during the buff..." but your own skills should count towards bonus damage.