He's walking very calmly towards the throne room, and his armour clinks against the cobblestone path. The time he had spent with the Mad King had rendered him half-mad, and burdened by the ever-increasing knowledge of dark secrets he must keep, he was at breaking point. A man of fifteen, and yet he felt no more than a boy unable to do the right thing in a city full of cruel, apathetic men.
He could not look upon the Queen without guilt. Could not sleep at night without hearing the Starks screaming. And when he learned of the King's plan to burn King's Landing and leave charred ruins for Robert Baratheon, he could not serve the King any longer without losing his sanity and losing everything that he valued about his knighthood.
But here he was, calm and composed, as if he'd fought vigorously in the war he had not been allowed to take part in and found that place of peace and serenity.
He was just as calmed and composed when he opened the door, walked into the throne room and instead of presenting his father's head as King Aerys has commanded, killed the Hand Rossart before ascending the stairs and plunging his sword into the back of one former King Aerys Targaryen.
It was only after Aerys' blood was dripping off his sword that he panicked. You swore an oath. He looked to the nearest exit and hesitated. Protect the King. He had killed the King. He, a White Cloak. The gods would have his head mounted on a spike if Robert did not. Protect the innocent.
He moved to slip away, but there were footsteps, and then voices, and then it was too late to salvage what was left of his honour.