It means that there was loneliness of the city as well as darkness in the night for the writer. ★「～のせい」=due to, because of 「夜の暗さ」だけではなく「市(=city)全体の寂しさ」があるという意味です。

My pleasure. It should be quite natural for learners of Japanese to think it difficult to read it correctly because even native speakers of Japanese could misread it. So, you don't have to worry about it.

Thank you for the explanation Ueda-san. I did actually read the whole story but I get confused/lost sometimes. :)

Not only the darkness of the night but also the atrocities of the ruler have made the whole city so lonely with few people on the street.

王様は、人を殺します。