Ambition, determination, cunning, and a bit of cleverness are all virtues of a Slytherin, but how do those relate to the genius, billionaire, playboy, philanthropist, Tony Stark? These virtues all relate to a different mask of him. Observe.
Ironman, though a Gryffindor in nature, has a few Slytherin qualities. Ironman has an ambition to do good, to protect. His plan, attack. Calculations pounding, scenarios created, possibilities considered, he uses his cunning to choose and integrate the best strategy to destroy the threat and neutralize the enemy. Ironman is the Slytherin side of Gryffindor.
The Merchant of Death was a sheltered Slytherin. His latest bout of creativity strived by determination creates the next point in weapons technology. After it is sent to be used on the wrong side, he celebrates his victory with his three friends; drugs, alcohol, and sex. The Merchant of Death was clever, but not enough. He was out-clevered by a friend, no, a father. The day Tony Stark discovered the betrayal, he defeated the Merchant of Death, but keep in mind, its not dead.
Tony Stark is a businessman. He must have great ambition to turn his dream of an empire into reality. Tony Stark is clever and merciless, perfect in his attacks; he cannot afford loss or failure in his company, his empire. Tony Stark never existed; it was always Obadiah Stane did. The public wanted a different Stark; so Stark Company gave them one, a ruthless Slytherin.
Tony, a broken man, a broken Slytherin. The day he was betrayed was the day he was created. Tony used his cunning to fool the guards into helping him to create the arc reactor and Ironman. He saved himself, but not his friend. In turn he left no survivors. Tony stopped the weapon manufacturing, Tony found out about Stane and used his two friends; Pepper and Rodney, to prove it. Tony killed Stane. Tony created a new world, or at least influenced it. Tony is the not-so-broken-Slytherin who decided to heal himself and those around him.
Tony Stark is a Slytherin. If you disagree ask yourself this, when Ironman said "I have a plan, attack", did you really think that he didn't have one?