Be a law unto yourself
Definition 1 of 2
Pro Tip 1/3Informal Usage
This phrase is casual, used mostly in spoken conversation or informal writing. 
My uncle doesn't care about conventions; he's a law unto himself.
Pro Tip 2/3Negative Implication
The phrase can imply someone is difficult to manage or control, not always positive. 
In meetings, Lisa is a law unto herself, often disrupting the agenda.
Pro Tip 3/3Unique Behavior
Emphasizes a person's unique way of doing things, differing from the group. 
He paints in the dark; truly, he's a law unto himself.