A man who's pure of heart and says his prayers by night
May still become a wolf when the autumn moon is bright

from Florence and the Machine – Howl Lyrics on Rap Genius

Meaning

Anyone, even the most pure of heart can turn into a “wolf” under the right conditions, into a more animalistic, possessive dude.

The last line is a well-known quote from the 1941 movie The Wolf Man

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