With due respect of all above opinions, it is highly possible for a person whether a superhero or not, to lose all sense of appropriate thinking, because he is a son.
In case of Tony Stark, He never adored the moments he had with his parents and in his twenties, he lost his parents long before his captivity by the Ten Rings in Iron Man 1, when he learned to drive his responsibilities correctly, also he missed his parents as well.
Now in Captain America: Civil War, when he came to know of his parents' death to be a brutal murder not merely just a car accident and the assassin is right in front of him (not his friend like Captain America) what should one expect him to do? After all he is still an Avenger, avenging his parents' death.
And about Bucky Barnes, in the post-credits scene Bucky chose to go into cryo-sleep because even he knows that he could be a danger to the society, a lethal weapon.
I think Tony Stark did what one has to do after knowing the truth. Really in person, after learning about his parents' death, everyone can give the same reaction. And I also appreciate Bucky Barnes for returning to Cryo-sleep, which is safe for him and for others as well.