Oh that's right I forgot the world government is not corrupt, they came into power and kill anyone who tries to find out the lost history. They throw people into impel down with no trial, just a freaking 3 headed judge who never agrees with himself, what a case. The people who kill an innocent man simply because he built the Oro Jackson and then the Pirate King sailed in it. The ciphor pol that used frankys ships to attack to judicial ship(ok this is really spandam's doing but still you think they don't know spandam is crazy, like they are blind or something?) The people who allow pirates to be pirates if they give them money, and keep other pirates in line. The people who refuse to tell the world Monkey D. Luffy took down crocodile, because it would simply make them look bad. Just lie to the public, afterall it's in the name of "justice". There are Marines who beleive in a true Justice. Smoker, Aokiji, Garp, Saul, Saul betrayed the government, Garp is a D, Smoker was effected by the Pirate King, and we dont know anything of Aokiji's story really. Also the D's are pirates who threaten the saftey of the world? We've seen 3 Pirate D's (one who wants to be a Shichibukai), 1 Revoluitonary D, and 2 Marine D's, so where again was it said D's are pirates?
Unfortunately, your arguement starts out weakly with an unprepared sarcastic note. They came into power 800 years ago, there is no specific evidence that they immediately killed anyone who researched the lost age. It could have been greatly debated, and in the end decided that it was in the best interests of the world to forget those 100 years.
They throw people into jail with no trial. I don't understand what makes you think they right to trial should be granted, they do not live in the United States. You may believe that is wrong, but others may see it as throwing individuals who knowingly became pirates, helped pirates, or commited crimes illegal to the government. The fact that Tom built the ship the Pirate King conquered the Grand Line with is very important, considering that we can safely assume Tom knew Gol D. Roger was a pirate.
I should hope that no government agent would turn himself in after he commits an unjust crime in order to bring his own sense of justice into play. The men working under him most likely have connections to him, and would also be in trouble considering they helped out. And yes, it is quite possible that they were blind.
Corruption is obviously present, it's human nature. However, that is only within certain individuals within the government. One must not judge the whole when they only know a piece.
I fail to see why not telling the world who truly took down Crocodile is bad. The fact is that he was taken down, and yes it would look terrible if the world found out that another pirate stronger then the shichibukai entered the grand line, because it could create an imbalance in the 3 powers of the world. It is the correct political choice.
You see those marines as the ones who believe in true justice, I see that as your opinion. Those individuals have little in common with each other and we have yet to see enough of them to judge them in any way.
"Where was it that said D's are pirates?" Seeing as how you mentioned that 3 of the D's are known pirates, I should like to think you answered your own question. Assuming however that you meant someone said that all D's are pirates, the person is obviously oblivious to the recent chapters.
Opinion is nice when you can back it with something other then another opinion. Touche my friend.