If I use Bellows Breath and damage a unit, then use Imperial Decree after, does that unit die?

No, the unit will not die from Imperial Decree.

According to RiftJudge FAQ #8633 and #9561, Imperial Decree only kills units that take damage after the card has resolved and its effect has become active. It does not look back in time to kill units that were already damaged before Imperial Decree was played.

Sequence Analysis:

1. You play Bellows Breath: You resolve the spell and deal damage to the unit. The event of "taking damage" occurs at this time.
2. You play Imperial Decree: Once it resolves, it creates a delayed triggered ability for the remainder of the turn.
3. Result: Because the unit took damage before Imperial Decree created its triggered ability, the trigger condition is not met. The unit remains on the board with its existing damage, but it is not killed by Imperial Decree.

If the same unit were to take additional damage later in the same turn while Imperial Decree is active, the trigger would fire at that time and kill the unit.