How to become a predator instead of merchant?

I believe you mean to change your class from the Trader to the Raider. To change your class you need to go to the Class hall building and tap on the class option. To do this at any time you need to buy a class change item.

You can do it for free though when you reach castle level 10, 13, 16, 19, 22 and 25. When you upgrade to these levels the game will ask if you want to change class for free. You can pick any new class.