Well my "prior knowledge" was not that much. I knew she was inside the building not just though the frikin' front door. And I have a tendency to have my weapon out and be in Sneak mode when going through any unfamiliar and/or hostile door.
Ah, see I was only expecting to find some feral ghouls inside so didn't take the necessary precautions. Of course, in the end keeping the scavvers alive is
Spoiler barely necessary since they betray you anyway,
but when I did it I was determined to keep them all alive, took me a fair few tries.
I sympathise with your disappointment though, sneak hit crits have a way of bringing even the ugliest nasty down to size. The legendary deathclaw was rather anti-climactic for me for similar reasons.