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.
That's also a possibility, I mean its not as farfetched as it may sound. Hadriel has absolutely no background but as somone said earlier he is actually an archangel. He could be a renegade angel that still resides in sanctuary from when Inarius created the priginal nephalem and took angels with him.
Yeah I just found it wierd considering he doesn't really say who he is, or why he is helping you/informing you about diablo. If he is known as an archangel you would think he would be on the angiris council or have some significant standing in heaven.
Im curious as to who exactly Hadriel is and what he was doing in sanctuary before the hero defeats diablo in diablo 2. I don't know if this question has crossed anyone elses mind, but throughout the whole game it is made apparent that Tyrael is pretty much the only angel involved (against the will of the angiris council).
I haven't read any of the sin war books so I don't know if he is mentioned in there. He could be one of the renegade angels who left the high heavens with Inarius. Purhaps we might see him again somewhere in diablo 3.