My understanding is the HaShem himself asked Avraham to sacrific Yitzchok. Why did an angel, and not HaShem himself tell Avraham that it Yitzchok’s physical sacrific wasn’t necessary. Also, is Yitzchok viewed in many ways as a sacrifice to HaShem and thus would have to kept in extremely pure conditions?
Although Avraham was commanded by G-d directly, Yitzchok heard the command from his father. Part of the belief in Torah is that we must obey and respond to the voice of G-d as transmitted by his servants, the prophets and the halachic tradition. Transmission through the angel strengthens that idea. OTOH, the Midrash (Tanchumah) says that Avraham himself complained to the angel that first G-d spoke to him directly, and now He sends a messenger, demanding that G-d speak to him Himself. G-d's oath in verse 16 is the response to this demand.