If I play Imperial Decree and then play Repeated Bellow's Breath on a unit with Guardian Angel, does it kill the unit or simply recall the unit?

Yes, your unit will be killed.

While Guardian Angel will save your unit from the first kill trigger by recalling it to your base, the second kill trigger from Imperial Decree will still find and kill the unit at its new location.

Sequence of Events:

1. Imperial Decree is played and resolves, creating a delayed triggered ability for the turn.
2. Bellows Breath (with [Repeat] paid) is played, targeting your unit.
3. Bellows Breath resolves:
   • The first instance deals 1 damage. Imperial Decree triggers, placing a kill trigger (ID1) on the chain.
   • The second instance (Repeat) deals 1 damage. Imperial Decree triggers again, placing a second kill trigger (ID2) on the chain.
4. The chain resolves in LIFO (Last-In, First-Out) order:
   • ID2 resolves first: It attempts to kill your unit. Guardian Angel activates as a replacement effect, destroying itself to save your unit. Your unit is healed, exhausted, and recalled to your base.
   • ID1 resolves next: It attempts to kill your unit. Because Guardian Angel has already been destroyed, there is no protection remaining. Your unit is killed at your base.

Key Nuances:

• Replacement Effect: Guardian Angel is a one-time replacement effect. Once it is destroyed to save the unit from the first kill trigger, it cannot protect against subsequent kill effects.
• Tracking: Imperial Decree's kill trigger does not require the unit to remain at the battlefield. It tracks the unit to its new location (your base) and kills it there.
• Damage Instances: Bellows Breath with Repeat creates two distinct damage events, which is why Imperial Decree triggers twice.

This is my interpretation based on the mechanics of replacement effects and triggered abilities. This interaction is consistent with RiftJudge FAQ #9419 and #9505.