Actually it was proving that being clever you can cheat the system. He showed, several time, why he wasn't as capable and probably put everyone on the mission in jeopardy because of it (Do they have contact lens replacements in space when no one else needs them? Probably not.)
That is one of the reasons why I love that movie, Ethan is the bad guy. Though that's probably not what the director was going for.
Ethan's character bothered me, too. The guy was hiding a serious heart defect, and while the viewer was meant to sympathize with his drive, it's not going to be much fun for his fellow crew members when a sudden infarc renders him unconscious and useless during some touch-and-go orbital emergency on the far side of Titan.
He was a 14 year old with a god complex who was arrested under suspicion of planning to overthrow the government, and proved them right when he attempted exactly that after escaping custody.
He and his compatriots were dealt with as any other traitors would have been while being caught in mid coup.
That is one of the reasons why I love that movie, Ethan is the bad guy. Though that's probably not what the director was going for.