When the triggered ability resolves, first the player whose turn it is (if that player is an opponent) chooses which creature he or she will sacrifice, then each other opponent in turn order does the same, then all chosen creatures are sacrificed at the same time.

None of the opponents are targeted. An opponent with hexproof would still sacrifice a creature.

If multiple creatures you control die at the same time, Dictate of Erebos’s triggered ability will trigger that many times.

If you control more than one Dictate of Erebos and a creature you control dies, each of the triggered abilities will trigger. Each opponent will sacrifice a creature each time one of those abilities resolves.

If you and an opponent each control a Dictate of Erebos and a creature dies, a chain reaction happens. First the ability of one player’s Dictate of Erebos will trigger, causing each opponent to sacrifice a creature. That sacrifice causes the ability of the other player’s Dictate of Erebos to trigger, and so on.