Rule 428.5.c

When one or more Units is killed due to a Cleanup, that kill action is attributed to the spell or ability that resolved immediately prior to that Cleanup that dealt damage to the Unit or Units.

• 428.5.c.1 The player responsible for the deal action is responsible for the kill action.
• 428.5.c.2 If the Cleanup that caused the units to be killed was the Combat Cleanup, the sources of the Combat Damage are…

Official Core Rules