Perhaps because she knew that wasn't actually the Barn of 1897, but the Barn of 1969...
Would that make a difference? Killing is killing. I don't let Barnabas off the hook the slightest bit for that event in 1897, I don't know if anyone else does.... I think Sarah of 1897 would sense her brother is back, killing someone, but since he wasn't doing harm before that, perhaps it's only action she's alerted by, not intent, and either way, she didn't have much of a chance to awaken and intervene given how fast things were moving.
I don't know how much Sarah (8 years old?) could comprehend about a 1969 brother inhabiting an 1897 brother. At most, I think she'd sense something strange and mysterious was happening with him, and if she got any real sense of it, this would take time, and she'd process and express it in ways we might find difficult to interpret. She's speaking from an alien perspective.
I doubt she'd care. I think she'd want to take Barney out to the woodshed.
Sarah and her perspective is one of the most interesting things about DS to me, now. Lydia probably agrees.