。
She crept into the room without a sound.
Literal
She [topic-は] footstep-sound [object-を] killed-and room [into-に] entered.
足音を殺す is a vivid metaphorical idiom literally 'kill the footsteps' — meaning 'silence one's footsteps / move stealthily.' 殺す (lit. 'kill') is also used metaphorically for muting other sounds (声を殺す 'silence one's voice'), suppressing emotions (感情を殺す 'kill one's feelings'), and even talents (才能を殺す 'waste talent'). The metaphor of 'killing' an emanation to suppress it is more general than English 'kill' would suggest.