I can offer a few options, all emphasised in bold:
Trust me. You don't want to get on his bad side.
The hidden face of Primark fashion
The Mr Hyde and The dark side
Mr Hyde is a part of the novel The Strange Case of Dr Jekyll and Mr Hyde, by R L Stephenson.
Wikipedia Jekyll is a kind and respected English doctor who has repressed evil urges inside of him. In an attempt to hide this, he develops a type of serum that he believes will effectively mask his dark side. Instead, Jekyll transforms into Edward Hyde, the physical and mental manifestation of his evil personality. This process happens more regularly until Jekyll becomes unable to control when the transformations occur.
Alter ego: the part of someone's personality that is not usually seen by other people
Many fiction writers, like Stephen King in his 1989 novel, The Dark Half, see the other self as a figure of repressed violence and evil.