I saw a lot of comments so I just like to explain my points.
Unfortunately we don't know any of Ruve POV of what happened in the first timeline in manhwa.
Because we can't know anything that Tia didn't see for herself, believed or felt.
We can't know what happened to people after Tia died, and we especially can't know how secondary characters felt.
Why?
Because the novel is told from Tia's POV.
What does this mean?
It means that the novel is entirely subjective.
We only know what Tia can grasp and how she interprets what she sees.
So can we know the truth?
NO.
We can only reconstruct the truth by confronting Tia's POV with other character's POV.
Does Tia's POV clashes with other character's POV?
All. the. time.
Jieun has a very different POV about what happened in their first lifetime. Even though Tia gets offended and refuses to listen to her (because she's stubborn and rigid - don't be fooled by the manhwa, the Tia from the novel is not as sympathetic as Tia from the manhwa), 1st Ruve's POV actually confirms Jieun's POV, and not Tia's. 1st Ruve was in love with Tia - it's only in Tia's imagination that 1st Ruve was indifferent to her.
Tia's vision of Allen is completely distorted. Tia's POV and Allen's POV are dramatically different. Like opposite. Tia doesn't know the true Allen at all, while Carsein finds out Allen's true colours almost instantly through his instincts. That tells something about Tia's weak abilities to judge someone's character. Especially when this person is lying to her.
I shall share 1st Ruve's official POV one day don't worry. But meanwhile, that shows what I know about 1st Ruve's POV before he went mad from the poison :
1st Ruve's POV portrayed Tia as the villainess.
Do you know what this means? He treated her as a villainess.
I'm going to play devil's advocate here. This is not in 1st Ruve's POV from the side story, but this is what I could reconstruct of his POV from everything I've read.
So try to imagine you're someone who was abandoned by your mother and unloved by your father. Your father however heavily dots on the girl of another man, your fiancée. The only person who has ever cared for you is the mother of your fiancée. You consider her your mother, even call her so.
One day, you're attacked by assassins when she visits you, but she protects her daughter with her body while she leaves you unprotected - that means she doesn't think of you as her son. You understant. But you can't help being wounded and feel rejected and not as loved as her own flesh and blood.
Later on she dies in your arms when her husband and her daughter are not at her side (where were they when she was sick and dying, honestly? She should have come first before anything else).
Did her daughter cried at her death?
No.
Because her daughter simply erased all her memories of her. She didn't pay respect to her mother a single time, and never asked about her because "one cannot miss what one doesn't remember" (that's what Tia said about her mother in first lifetime) and always acted cold and emotionless around you.
While you were grieving the death of your adoptive mother, a kind old man showed up and comforted you. Duke Jenna.
You trusted him and cared for him. He warned you against the Monique family. Aristia la Monique was a very cunning girl, and she was using you to put her son and her family on the throne. The whole imperial faction was behind her.
They thought you were an incompetent person, and they wanted her on the throne instead of you. For her, you have no value at all. She too despises you. She only needs your seed to get what she wants. If she somehow learnt that part of your blood was not noble, she would refuse to be touched by your vile hands. She too has commoner's blood, but you'll never say a word of this, because that would hurt her mother.
For her mother, no one shall ever know the truth. If only your fiancée wasn't acting so proud. This pride of hers is a joke and it grates on your nerves. You lose your temper more and more lately, you don't seem like yourself.
You can't really recognise yourself sometimes. You can't sleep either. This is taking a tole on you. Your life is only about duty and work. You have to wear a mask constantly. You don't have a single person to confide in. It's suffocating. No one is on your side. Only Duke Jenna.
One day, a strange girl has appeared in the royal gardens. Jenna came to tell you that the prophecy has been wrongly interpreted and this girl is the one God had meant to be yours.
She's willing to marry you she says, and seems so open and genuine. She clings to you as if you're the only safe anchor in this world she knows nothing about. She needs you. This lovely, warm-hearted girl needs you. You decide to protect her. From the nobles, from the imperial faction and the Monique family's schemes. She says she loves you. Surely, what you feel for her must be love too. You don't know what love feels like, as you were never loved before. You'll try hard to love her too.
Maybe this is your chance to escape. Escape this loveless marriage with this cruel Monique girl, escape the control of the imperial faction, and get the family and love you long for. You will not be a puppet king. You'll make the Monique girl your Queen. It is the honourable position of the Emperor's beloved and the Crown Prince's mother. That way, you won't offend the Monique family and you can keep the imperial faction on your side, without being controlled by them. It won't be easy with the Monique girl's pride. Surely, she'll try to hurt Jieun. Maybe she'll even scheme to get rid of Jieun. You'll have to be careful and put the Monique girl back in her place whenever she tries something suspicious.
She was never meant to be the Empress, but she'll claim otherwise. She's been trained all her life for the position, so she won't step back quietly. You'll have to watch her, so that she can never hurt Jieun or you. But surely this is the best decision. God has sent Jieun to help you. You shall be a good Emperor and with Jieun by your side, as the prophecy foretold, you shall both bring peace and prosperity to the kingdom.
See the trick ?
This is called subjective writing. You empathise with a character and see the world only through his/her eyes. Their ennemies become yours, their ideas and insights become yours. And it's heavily used in AE, more than in other novel/manhwa. Hence the strong reactions against 1st Ruve, and the very mild reactions towards Allen.
As hard as it is, we have to remember that there is some truth to 1st Ruve's POV apart from the lies he was fed by Jenna. Aristia was cold and emotionless. She indeed never cared for her mother after she died. She didn't trust anyone, and had no friends. She even was in a very cold relationship with her father, and an even colder relationship with her fiancé. And this was all her own doing and choices (as second lieftime proved without a doubt.)
She also never openly expressed 1st Ruve any bit of love or care. She only loved him from afar. And this is why 1st Ruve never found out about her feelings before it was too late. And misread her entirely all her life. Tia didn't care what people thought about her at all, and put on a prideful attitude to keep people at bay. This was her way to protect herself, but this ultimately became a weapon for her ennemies.
Am I saying this to justify the way 1st Ruve treated Tia? NO. Never.
But the way 1st Ruve treated Tia can be explained (I didn't say "excused") with his perception of her, and the poison he was fed. 1st Ruve slapped her, but Tia was fed the same poison and she stabbed him. Which resulted in her trial and death sentence by her peers. 1st Ruve was actually hoping her father would plead in her favour and save her life. 1st Ruve never killed Tia. Nor Keirean. So this is unfair to accuse him of doing so.
Stabbing the Emperor meant death. And what pushed Tia to stab 1st Ruve is not the mind trick her jealous, desperate and mad husband invented to make her reveal her true feelings, but the poison she was fed. As much as Tia is innocent of her stabbing 1st Ruve, 1st Ruve is innocent of hurting Tia due to the poison. Either they're both innocent or both guilty. Choose your poison (lol).
Ruve is not the villain of the story. And 1st and 2nd Ruve are not the same person. Just the same soul.
Ruve and Tia are soulmates. That's what the author wanted to convey. They are destined to fall madly in love with each other, despite their own will. In first lifetime it's 1st Ruve who's unwilling. In second lifetime it's Tia. But in the end they are destined to fall for each other. And this has nothing to do with any prophecy. They both swore they wouldn't love each other, but love doesn't obey to reason or will.
If you want to be convinced that Ruve is a good ML, then just read the novel or the manhwa to the end, and then see what you think. I was convinced personally. But I know that some people will never be able to get over their first impression of him, because 1st Tia's POV is too vivid for them and they've identified too much with 1st Tia.