。
She held her breath.
Literal
She [topic-は] breath [object-を] killed.
息を殺す ('hold one's breath, suppress one's breathing') is part of the wider 殺す-metaphor family — 'silence' an emanation by metaphorically killing it. Compare two close-but-distinct breath idioms: 息を殺す ('hold one's breath' — for stealth or suspense) vs 息を呑む ('catch one's breath' — for sudden shock or admiration). The first is deliberate concealment; the second is involuntary reaction.