I think Nathan is getting a raw deal here. While he is a pompous ass, we really shouldn't expect him to act like any of the others with respect to his powers. The people in the show that embrace their powers are the ones who are 'freaks'. Claire and Nathan are approaching the problem like I think I would, they are trying to keep it hidden and live like normal people. Nathan admitted to Peter that he knew he could fly, but he also made a good point, that that realistically that ability wouldn't be useful to him and thus he doens't care about it. As to ruining the painting I guess he just wants Peter to stop wasting his life on unrealistic pursiuts and do something useful, which makes much more sense than supporting the notion that a guy travelled back in time to warn him that the world would end if he didn't save a cheerleader.
From our perspective Nathan is a jerk, because he is not acting like a hero when he has heroic powers, but from his perspective, even with the knowledge of his ability, it's preposterous for people to run around like super heros trying to save the world. That just isn't reality for him and he won't accept it.