You've picked my curiosity Actually it is really good question: Why was Hadriel before the Chaos Santcuary? In terms of Gameplay we know that he was just pointing us that Diablo is protected by five seals. But lorewise how could Heaven know that a mortal is walking on the Hell plane? Maybe they have some agents there. Does Tyrael has any allied Angels that are rebels like him? Maybe Hadriel is one of them. In Diablo wiki it is said that Hadriel appears in Hell as a projection.