Luffy thinks the Pirate King is the freest person in the world and the one thing he can't stand is when another person takes someone's freedom away. Be they marine or pirate. That's why Luffy is no hero. He does what he wants based on selfish desires and if he happens to do some good then that's just a bonus. That's why his crew is loyal to him, not out of fear but respect, love, loyalty and friendship. Things that no money can buy and no prison can chain.