This is the set-up: HE player has two Keeper of the Flame units in his Kingdom zone. The DE player has a slave pen out. DE plays Soul Stealer onto one of the Keeper of the Flame units.
My understanding is that you cannot affect cards that are coming into play except with abilities that specify "just played" or "being played" per the FAQ. So neither Keeper of the Flame can use his ability to destroy Soul Stealer while it is being played. So the card comes into play and its text effect is resolved, i.e. the Keeper goes over to the DE player.
Now, the action priority would pass to the HE player, who uses his second Keeper to start a new action chain by destroying the Soul Stealer. In response to that action (destroying Soul Stealer) the DE player uses Slave Pen to sacrifice the Keeper of the flame and due to LIFO, the Keeper dies.
Is that the correct way to resolve the play?