The Quatermass Xperiment (1955)

A missile, launched by the team led by Prof. Quatermass, lands in the English countryside. Of the three members of the crew, two have mysteriously disappeared. The third one, barely alive, undergoes an horrible metamorphosis turning into a monstrous "thing". When he breaks out and, chased in vain by inspector Lomax, starts killing humans and animals to feed his transformation, Quatermass realizes that this is the way chosen by an alien form of life to invade the Earth.