meaning - What do you call someone who decides to do something even though they know its wrong

As a native American English speaker, I would colloquially refer to the person you described as a "rebel."

Oxford dictionary's second definition for Rebel fits my usage:

Rebel
- A person who resists authority, control, or convention.

Another word that came to mind in the same context is "contrarian." Oxford dictionary's first definition for Contrarian sounds like a good fit as well:

Contrarian - A person who opposes or rejects popular opinion

If we are talking about a person who is willfully breaking the law, then "scofflaw" is a good fit. Oxford dictionaries defines "scofflaw" as follows:

Scofflaw - a person who flouts the law, especially by failing to comply with a law that is difficult to enforce effectively.

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