@Timjer We all saw what Sakazuki did to Ace, and just like him, the Admirals and the whole armies of Marines at their command are destructive, dangerous, powerful, relentless, and ruthless. They'll stop at nothing to bring all pirates to justice.
The Marines and Cipher Pol are willing to kill hostages and innocent bystanders and will often obliterate entire islands or large groups of people just to take down a relatively small group of people. They use justice like a blind hammer and anything they cannot control they perceive as a threat that deserves total obliteration. What’s ironic is that the Straw Hat Pirates have actually solved many problems the World Government either caused, ignored or escalated yet the whole world believes the Straw Hats are bloodthirsty maniacal terrorists because of World Government propaganda.
