Rulings

If an Aura you control is destroyed at the same time as the creature it enchants, Hateful Eidolon's triggered ability counts that Aura to determine whether it triggers and how many cards you'll draw.
If Hateful Eidolon dies at the same time as one or more enchanted creatures, its last ability triggers for each of those creatures. This includes Hateful Eidolon itself if it's enchanted.