Arthur couldn't quite explain his initial reaction to reading his adoption form. The first thing that hit him was a wave of disbelief. Then it was shock, followed by rage.
As he held the file in his hands, he wanted to rip all those papers apart. All the articles and images depicting his abuse were too overwhelming to withstand.
When he first found Penny's letter to Thomas Wayne, he couldn't believe his eyes. A secret affair with a billionaire was the last thing he would've expected to discover about her. Of course, he always knew about her obsession with the guy since she never shut up about him. But when Penny seemed to confirm the confession, Arthur was repulsed by the fact that she hid such an important link to his identity for so long.
Despite that, he was elated to discover that he actually had a real, living father. So when he was turned away at the gates of Wayne Manor, he didn't understand. Why did that butler call his mother crazy?
Furthermore, why did Thomas say the same thing? Arthur was convinced that he finally found the lost connection he'd been missing the whole time. His mother wasn't delusional. She couldn't be. Arthur would not allow it.
Then again, he always felt disconnected from her. Something in their relationship was missing. She wasn't ever present for his needs and never took on a true motherly role.
It dawned on him that Penny was the root of all his problems. She wasn't related to Arthur at all, and because of her neglect, she was clearly responsible for his mental illnesses. He was caught up in the biggest joke of all time, and Penny's devastating lie was the punchline.
As Arthur stared down at her on that hospital bed, hatred coursed through his entire being. Penny deliberately held the truth from him back then, and her stroke literally caused her to be unable to speak now. There was no point in keeping her alive.
Arthur nearly enjoyed holding that pillow over her face. He no longer wanted anything to do with her. He thought it was only fair to suffocate her for what she did. A part of his brain unlocked after he did it, though. He didn't know it at that moment, but in due time, a whirlwind of dark memories would come rushing back to meet him.
Arthur punched the walls of his padded cell in a fit of rage. He kept punching, but he didn't get the satisfaction of truly hitting something. He let out a resounding yell mixed in with a bit of senseless laughing.
He clutched at his hair in an attempt to calm down, but he couldn't relax. He wildly screamed until his throat became hoarse, and tears streamed down his reddened face.
Some orderlies and medical assistants came in after being alerted by the noise. Seeing how hysterical he was, they tried to get Arthur to stay still. He resisted, so they pinned him to the ground and injected a sedative into his arm. He immediately fell asleep.