She doesn't seem too freaked out in the flash backs. Sure, cut to the present it's disturbing, but in the flashbacks above there's no indication Cass is the least bit concerned about having been a part of a murder.

I don't recall them stating that her first murder was Cass' first encounter with death, but really, it wouldn't matter. What made her first killing so traumatic was what she 'read' as the person died.

Here, it's a quick shot and done. No reading possible

It's a lousy scene sure, but doesn't go against Cass' origin in any way that can't be handwaved away.

Beechen, however...