Based off of the 2003 anime, which I do not own. This doesn't really make a big difference though, only for a few details, so if you're a Brotherhood/Manga fan, still go read ahead.
"Dad?"
Hohenheim turned in the chair, surprise creasing his features.
"Ed? What are you doing up?"
The golden-haired boy gave a massive yawn.
"Nothing. Couldn't sleep. What're you doing?" His words were slurred from sleepiness, and Hohenheim smiled gently.
"Ed, you should go back to bed."
"Alright." He yawned again and rubbed his eyes with his wrist. He turned to go, then paused at the doorway.
"G' night."
That innocent smile, the innocent gesture of saying goodnight to his father warmed Hohenheim's heart. His smile softened, and even though his son's back was already turned, he whispered his reply, a gesture from father to beloved son.
"Good night, Ed."
Dad had left.
Why? Why was he gone? Mom said he'd come back soon, but he never did. And she got this far-off look whenever we mentioned him.
Why had he left? Why had he left without saying goodbye?
There were suddenly tears running down my cheeks, and I viciously wiped them away. Crying wouldn't bring him back.
"Dad?" I knew he wouldn't answer. Couldn't answer. He was too far away already.
"Please come home."
Mom was dead.
It was his fault. He never came home. Maybe if he had, he could have helped her. Taken her to a doctor before it was too late.
He hadn't even come to her funeral.
"I hate you," I suddenly said, angrily. "Why did you have to go? Why did you have to leave us?"
That's right.
It was all his fault.
His fault that Mom was dead.
The bastard had come to Risembool, greeted me like nothing had ever happened. As if Mom wasn't dead. As if he hadn't abandoned us.
Bastard. I hated him. Hated him and his arrogant, pompous, gentle face.
Why did he have such love in his eyes? Why did he look at me like that, like I was his precious son? He had given up rights to that long ago.
"Bastard," I muttered, and let out a soft chuckle. A bitter chuckle, full of unsaid irony. "You lost your title as 'Dad' a long time ago."
When Mom died.
I tried to find comfort in the lonely stars twinkling in the empty blackness above.
Hohnheim stood in front of the burned remains of his home. The home that he and Trisha had built together, had raised two beautiful children together.
He sighed.
"I'm sorry Trisha... I had been too much of a coward to stay with you and our sons."
There was no reply, but he hadn't expected one. He took one last long, regretful look at the ruins of his past. Of all his joyful moments and most loved times.
"Goodbye."
He left, and never came back.