The phrase is an "indication that it is night". It serves as a metaphor for the absence of moral oversight or divine protection during the darkness of the night, where "innocence is juxtaposed with increasing guilt". It contrasts the safety of his sleeping daughter ("the angel") with the atrocities he is committing outside.
Below is an overview of the 2018 film to help you prepare your paper. Cuando los ГЎngeles duermen
The film is a psychological thriller that explores the moral decay of an "ordinary man" pushed to extremes by a single accidental event. The phrase is an "indication that it is night"