。
She was hiding behind the door, holding her breath.
Literal
She [topic-は] breath [object-を] killed-and door [genitive-の] shadow [in-に] was-hiding.
息を殺す ('hold one's breath, suppress one's breathing') is part of a wider 殺す-metaphor family — 'silence' an emanation by metaphorically killing it, the same logic at work in 足音を殺す ('silence one's footsteps') and 声を殺す ('silence one's voice'). ドアの陰 ('the shadow / behind side of the door') uses 陰 ('shadow, hidden side') for the concealing-position sense. 隠れる ('hide [intransitive]') in past resultative 隠れていた marks the sustained state of hiding — paints a tense scene of someone dodging detection.