Safe, it always came back to being safe. That one word had become the goal just out of reach for so long, then given to her in amounts she hadn't been able to handle at first. Now, anything that was a threat to that safety was a serious threat indeed.
That look from her dad spoke more than all the words that would be necessary, reminding Claire of all he'd done, to himself, to others, for her. Not everything were things she wanted to remember, painful in all sorts of ways, but it reaffirmed what he was saying.
It made the question that lingered, wondering if she was a bad person for what she'd done and the lack of expected remorse – she hadn't wanted to kill a person, but she wasn't sorry Jareth was now gone – fade off in her mind, because voicing it was ridiculous. She didn't think her dad was a bad person, or, for that matter, anyone else she cared about who'd had to kill in their lives, so question her own good or badness would be stupid, right? Still, the look on her face likely told she wasn't quite done questioning herself.
She knew the answer to his question, but it still was hard to say 'yes, I could kill someone again' out loud, so she nodded.