Because the navy is here only because the WG is here. The WG after the fall of the great Kingdom managed with the most powerful families /20 kingdoms to create Peace (and with Im sala or special weapons). Thanks to that pact the world was govern and balanced The navy is only create to keep the WG from being attacked by others (pirates/revis etc) and handle day to day problems in the world. The true powers of the WG don't lie in the navy. If the WG falls without an other political power to replace it there will be no more navy.
If you want This marine to keep going you have to turn a blind eye for the horrendous things that happened.
I agree with what you said but what garp is trying to do to balance things out? He is trying to go on with an institution which is getting more corrupt day by day. What good he will achieve by carrying on like this?
He is supporting or turning blind to acts of horror(WG acts) to stop preventing the rise of another act of horror (pirates). And, in the end he isn't succeeding in checking either of them.
He is the only D in the family who hasn't shown traits of being a D and cherishing freedom.
To me, the only reason that his portrayal is so conflicted could be due to the fact that he might be knowing about one piece and void century