The death of Herod was not the sign for Joseph to return to Israel. Rather, it was the angel’s command, which also speaks of others besides Herod: those [in the plural] who sought the child’s death.
God had spared Jesus from an early death at anyone’s hands to allow him to complete his task.
No, the death of Herod is not the sign, but rather the vision that occurs following the death of Herod. Perhaps this appearance only occurred sometime after he had died. The angel speaks in the plural about the death of those who sought the child’s death
(Matthew 2:20). Apparently, Herod was not the only person who was looking for Jesus for the purpose of killing him. We do not know who the others were, but it is clear that they died either shortly before or after Herod. This is when the angel provides the signal for the entry. This shows that the time for Jesus’ task in Israel has now come and that it was now safe for him to return, for there was no way God would allow him to die before completing his task.1
19 But when Herod died, behold, an angel of the Lord appeared in a dream to Joseph in Egypt,