CONTENT WARNING: This deals with the events surrounding Zuko's death sentence and Azulon's murder. There are also mentions of child sexual abuse. None of the contents are graphic, but reader discretion is advised.

This is my own take on what went on behind the scenes during the flashbacks in Zuko Alone. It should more or less fit within animated canon. (I don't care for the comics.)

She did not knock on her lord husband's doors. They no longer had anything resembling intimacy, but the rule they'd had at the beginning of their marriage still stood - they had free reign of each other's rooms. It was the first time in ages she'd had any reason to do so.

The vast chamber was unlit. Ozai, his hair down and his robes askew, was rummaging through a drawer, holding a small flame in his palm. He had not heard her enter.
She stepped closer, the sound of her footfalls swallowed by the rug. Still unaware of her presence, Ozai retrieved and examined a slender glass bottle. She recognised the medicine he took when sleep eluded him.

'So tell me, Ozai, do you intend to sleep on it?'
He nearly jumped at the sound of her voice. The flame in his hand flared then sputtered. She continued: 'Or do you intend to pour the contents of that vial down Zuko's throat?'
He let out a sigh. 'How do you know?'
'Your daughter eavesdropped and then proceeded to taunt her brother about it. That is how I know. How could you not tell me? Did you just intend for me to wake up and find my son dead in the morning?' He looked down, said nothing.

She walked around the bed and up to him. 'You won't even look me in the eye and say what you intend to do?'
'I don't want to do this, Ursa. But there is no other way.'
'Are you really such a coward?' She turned her back to him. 'You disgust me, Ozai.'
'We have no choice."'
'We? It was you, you with your ridiculous ambitions, that dragged us all into this mess. And now you won't stand up for your family?' She started pacing the length of the room. 'In private, you denigrate the Fire Lord, you complain at length about his age and his senility. You do it so often that Azula can repeat your like a trained monkey-parrot. But when it comes down to it, you're too craven to face the man.'

'Ursa', he breathed. There was a catch in his voice, something she'd rarely heard before. She turned to look at him. His eyes were wide, and, she realised, frightened. 'Ursa, do you know what Father does to anyone that defies him?'
'He already wants to kill Zuko. What could he do that is worse than killing an innocent child?'

He said nothing at that. She could hear his loud breathing, strained and shallow. She had never seen him so close to panic before, not in a waking state. Finally, he asked: 'Don't you understand? Zuko's life is already forfeit. He has decided so. Killing him myself is the most merciful thing left to do.'
'You call that mercy?'
'Yes, Ursa! Zuko will die regardless! The only mercy we can give him is to give him a swift and mainless end, because my father will not be so kind. And… if we defy him, if we don't bow to his will, his anger will fall on all of us, not just Zuko.'
'So you would rather have Zuko pay the price for your mistakes? I would bear the full brunt of his anger to spare my children, die a thousand times over to save them, and you should feel the same. It disgusts me that you do not.'
'You don't understand, it's nothing to do with- with how I feel, it's… the Fire Lord, his will is law, his orders have been issued, there is no going back. If we defy him…'
'You are pathetic, Ozai. You would kill your own child in cold blood just because you are afraid of standing up to your father? It sickens me to see how low you will sink to please someone who will never be pleased. Aren't you too old to beg for scraps of approval from a father that will never love you?'

Ozai looked at her with a stricken expression. For a while, there was only the sound of their panting - hers in rage, his in fear. He took a step back and sat, or rather fell, down on the bed.
He did not look her in the eye as he said: 'I am not fool enough to think he will love me if I obey him. But I know what he will do if I don't.' His hand was pressing on his thigh, where she knew his old burn marks were. 'I told you how I got the scar, didn't I ?'
'If you're trying to make me feel sorry for you, Ozai…'
He looked up at her, eyes wild and despairing. 'I was an insolent child, and he ordered twelve of his most loyal soldiers to teach me respect. I was sixteen, and I only barely survived it. Zuko won't. Do you understand now?'

She could only stare. She wanted to believe he was lying. She knew him well enough, by now, to understand he was a creature of deceit. But she had also witnessed his restless sleep and strange rituals and the tense awkwardness of their wedding night. She knew then that he would never defy Azulon; that for all the resentment he harboured, he would never act on it.

But she was not Ozai.

Faced with nothing but bad options, she would not back down from making a choice.

She lifted her chin and said: 'You are right. Azulon will not let our child live.'
'Yes. You see, don't you, that a Fire Lord's decree—'
'— Can only be revoked by a Fire Lord.'
He looked at her uncomprehendingly. 'He will never listen to you, Ursa.'
'I do not mean to speak to him.'
He blinked at her, still not understanding. She went on: 'If you are unwilling to sacrifice yourself for Zuko's sake, then I will do what you cannot. Why, you could even get a throne out of it.'
He gaped at her. She fought the urge to slap him. 'Azulon is old. Iroh is away, no one knows where. There is still an opportunity to grasp that throne you so desire, if you have the courage to make a move. And I will aid in that, not because it is a good option, but because it is the only one.'

He shrunk on himself. His breathing was laboured. 'I cannot… even if he wasn't my Father, he is my sovereign.'
'A sovereign that will turn on his people, and a father that turns on his children. Have you forgotten your lessons on government?'
He looked down again, hair sliding across his face. His voice was barely above a whisper when he said: 'It was Father who told them to leave a mark. He said the lesson needed to sink in.' He closed his eyes and quoted: 'If you show me no benevolence, I will owe you no loyalty.'
'Yes.'

A pause. Then, louder: 'I will help you do it.'
'No. You stay here and do your little calligraphy exercise. I can't risk you getting us caught, and that way, you will be able to deny knowledge of what I did.'
In truth, she doubted he could accomplish it. He would get cold feet, or do something foolish that would give them away. Despite everything, and everything was worse than she'd thought, she doubted he could bring himself to defy his father, not when he'd already paid such a price for it.

She told him: 'One of us needs to take it upon themselves to act, no matter the consequences. I will bear the punishment for regicide, so long as Zuko stays safe. I am ready to exchange my life for my child's.'
'No.' For the first time this evening, he sounded sure of himself. 'If we succeed, and I become Fire Lord, it will be my duty to mete out punishment. You will be banished.'
'Regicide means death, and you would reduce it to mere banishment?'
'I know you cannot stand the sight of me anymore. If you want a way out of this marriage, I am giving you it. Take it, and leave the country.'
'Why so kind an offer, Ozai?'
'You are giving me a crown. And I... I do not want you dead.'
She supposed it was as close to a declaration of love as Ozai would ever get. But she felt nothing but revulsion for him now.
'I am not doing this for you, Ozai. Zuko is my only concern.'

He let out a shuddering breath, that, if one listened closely, could almost pass for a sob. There was silence for a while before he said: 'Of course.'
The self-pity in his tone revolted her. It was only his cowardice and self-regard that had ended their marriage, it was his blunder that threatened Zuko's life, and here he was, still feeling sorry for himself.
She blurted out: 'You chose your ambition over the safety of your own family. You are fortunate that I am here to repair your errors. I will give you what you desire above all else. For you, the price I ask in return should not be too high.'

He looked up at her, eyes bright with something indecipherable. 'What is your price?'
'After I leave this room tonight, you will never see me again.'

I welcome all comments!
You can also reach me on Ao3 (same username) and tumblr ( peregrinvs dot tumblr dot com)

NB: I prefer to write Ozai and Ursa both as complex characters with motivations beyond "being nice" and "being evil". That does not mean I as an author adopt the characters' point of view or condone/excuse their actions.

The line about benevolence and loyalty is from a Chinese idiom I read long ago, but I no longer recall the original expression; if someone knows anything that sounds like it, I would welcome the information.

I hope youe enjoyed the story!