"What's her stake in this? Why is she helping Marcie?" How is she helping Marcie was another question he didn't ask - two questions in a row already felt like he'd asked too much, any more would be broadcasting his ignorance.
Is she only interested in Marcie to get to me? That was the real question. Does she know? And if she doesn't know I killed her, and she finds out, what then could she do to Marcie, or himself?