the dark side of (someone or something)
The negative or troubled part of someone or something that is usually concealed. My mom is always sweet to my friends, but I've seen the dark side of her enough to know mean she can be. Shady deals like this are the dark side of the music industry, unfortunately.
dark side of someone or something
Fig. the negative and often hidden aspect of someone or something. I had never seen the dark side of Mary before, and I have to tell you that I was horrified when she lost her temper.