Delayed emotional reactions are very much a real thing.
With respect, in the real world, that is true. But this is television. This is
telling a story, which is not the same as "depicting the way it is in the real world in realtime." Stuff has to appear on screen for it to be relevant. They have ways and tropes for telegraphing, "this character is so shocked by events that they have no real response yet."
And taking jewelry off the dead body (with a
satisfied look iirc) isn't among those ways.
Edit: I just re-watched the scene. She starts with claims she knows nothing of the crimes
1. Then moves to claiming she didn't break "your rules"
2, but merely bent them. Then, after everyone but her mother is dead, she retreats to, "I can be good," effectively recognizing that she has not been good to this point.
I don't see how this gets read as her being on teh right side of morals or ethics, or secretly a good (or even okay) person. I can only take it as a deep expectation that the writers will pull a "Gotcha!" moment for Agatha, because none of that is present in the text to date.
1. Which includes taking magics above her age and station,
and practicing the darkest of magics - so it isn't that she hasn't actually done anything yet. She used forbidden stuff.
2. Thus, positioning herself as separate from the others from whom she's been learning, which traditionally is an assertion that she's above or beyond the rule of her peers.