That just means Hawke is a another cog in the corruption, not that Hawke isn't corrupt. You see similar rationalizations for corruption in, say, Afghanistan, where bribes and kickbacks and patronage networks are the established way of doing business.
When you get down to it, pretty much every member of Hawke's crew has at best a casual acquaintenship with the concept of rule of law. I'm not just talking how Isabella is a pirate with a not so distant criminal past or how Varric blackmails and bribes with the best of them, or even that Anders is a dissident to an established authority and is helping a subversive network of dissidents: we're talking about a group in which even the captain of the city guard and a Chantry boy is amiable to going around and participating in break ins, burglary, and outright murder with little more than a tisk tisk. A group led by a person who is a nouveau rich patron, who effectively purchases nobility status and who's influence is used as shelter and protection by the companion crew of accomplices for their own ends. A person whose closest thing to a legitimate business is cooperation with foreign financial interests paying the local mafia.
If we weren't the ones playing them, Hawke and co would be the kind of enemies we'd be expecting. A metoric ascent from nobody to nightmare via a good deal of timely violence? The corrupt and complicit city guard captain who looks the other way for all his activities, when she's not taking part? The fem-fatale pirate and past slaver who's moving to other sorts of crime? The 1% business partner and spymaster who buys friends and influence? The underground revolutionary? An enhanced bruiser who has no job or skill set past being the muscle? The foreign magi who was exiled for continuing down a path of mad magic-science? A complicit churchman?
It's quite telling that the people who most consistently fought against corruption, and even then primarily in her own sphere of concern, were the primary antagonists.
Dragon Age 2: Corruption is Cool.
Yea sure, lets be totally inaccurate about a man's history and only see that he is a rich guy with the captain of the guard in his pocket. And of course the system is always right with no margin for error and laws in medieval times were fullproof and totally fair.
Go and check every individual quest in the game, and other than working for outlaws to enter the city and fighting the Templars in 2 instances, if he chooses to, everyone else he kills is either in self defense, slaver, bounty, or an abomination. Sometimes he operates outside the law but killing slavers in Hightown is hardly "corrupt".
He gets meddled in politics because the Viscount himself asks him to in act 2 and in act 3 there isn't even an established power in Kirkwall, just the military rule of a madwoman that most highborns don't accept her. She is effectively is an usurper.
Doing the wrong thing for the right reasons is hardly "corrupt'. He even accepts responsibility for the actions of his friends when he really can do nothing to prevent the events that occur short to kill them on the spot (and that's mostly Anders).
Even if you play the game as a total thug, taking in face value his actions, he does more good than bad. He certainly is not a saint and he can be a pretty glib killer and a really bad person if the player chooses but if I needed someone to blame for the misfortunes of the world, I don't think that would be Hawke although politics would make him a great target cause that's what politics are for.
That said, he should answer for his trespasses but ultimately I believe he is a hero.
PS. Of course I understand that this is a game and has huge plot holes, but that's the story.