You can reframe it as a moral question pretty easily. Is it wrong if you(not you you, hypothetical person in this quandary for clarity) kill two kids, if you know the room they are in will have all the air sucked out of it soon. I'd say yes, and argue that it's a similar, if not the same situation. Plus, the reason Agatha was threatening to kill them was so she could steal Wanda's power for selfish reasons, so you can't even argue it is an ends justify the means kinda thing.