No, she doesn't fully grasp why he's angry. She'd expected some disappointment, a certain amount of discomfort, but not this. Can't he see that she wanted to save his life as well?

"I couldn't!" she bursts out, incredulous and slightly indignant, and it even sounds childish to her own ears. There's a pause while she sets her mouth in a thin line and has a brief internal debate, then, "It wasn't your burden to bear."

It's sincere, with an underlying stubborn edge.