be a law unto (oneself)
To do whatever one desires and/or ignore rules and regulations. If you need to delegate, I wouldn't ask Gina—she is a law unto herself, and it's hard to persuade her to do anything she's not interested in. How many times has Jason been arrested now? Geez, he sure is a law unto himself!
be a law unto yourself
behave in a manner that is not conventional or predictable.