Why do Kagamine len and rin sing such dark songs?

The Kagamine twins, Len and Rin, do not sing exclusively dark songs. They have a versatile repertoire that covers various genres and themes, including upbeat, cheerful songs, love songs, dance tracks, and many more. While they have sung songs with serious topics or somber moods, it is not representative of their entire body of work. Additionally, songs' interpretations can be subjective, and what one listener perceives as dark might be different for another.