Zealot Barbarians at higher levels become immune to death to a certain extent. You can knock them down to 0 HP and they can still keep fighting. But at that point you're at a fairly high level and level 1 spells like sleep should be completely behind you. But the zealot barbarian, despite being partially immune to death at that point, has such a low amount of HP that they're vulnerable to the sleep spell. While killing a zealot barbarian becomes difficult in that state, you could put them to sleep with a simple level 1 spell. Their rage ends and then they're dead.