Now that we've seen Senadina's story to the presumed end with her death, it's pretty clear that she's... basically just a person who did her best, but had her own flaws, desires, and trauma. Far, far from a villain.
Senadina was someone with a pure goal. She just wanted everyone to live happy and free, she was utterly selfless, and heartbroken about every selfish decision she ever took, even ones purely linked to her own survival. In other words, yes she was selfless, but it was to a toxic degree. Selfless and hopeful past reason, to a point some might call insanity. Faithful.
And because of it, because she was a beacon of faith, Senadina was completely unable to live. There were always rules. There were always expectations. There were always hardships. Everyone was always looking at her, wishing for happiness, but burdening her all the same with that love. She couldn't afford to rest, couldn't afford to fail, despite failing over and over again and watching the people she wanted to protect be destroyed mentally and physically. She had to keep trucking along even when everyone else gave up.
There was hope, but it was resting at the end of a path she couldn't see the end of, and she could only keep walking in the dark, knowing there would be light when she reached the end of the tunnel.
By the time she lived on Mars, Senadina was living on fumes.
So why did she fall for Entropy? Because she's a perverted parent who wants to date her child when she's older? Of fucking course not.
It's because Senadina was still human, with the same desires as all those people she wanted to protect. Desires for carefree freedom and happiness.
Entropy came from a future where Senadina doesn't have to be everyone's princess/god/mom. The Senadina Entropy fell for was an innocent girl her age, an equal partner who went on adventures together with her. Meeting Entropy made Sena dream up that alternate life where she's happy, no power dynamics with the people she loves, just equal partners who can be carefree and heartfelt and loved without strings attached.
The other Sena is the crystallization of that wish. She could never be her, but she is still her. A path that was entirely impossible. A path that was everything the Senadina at the end of her rope wanted.
A fantasy of just being a normal teenager who does normal teenager things like holding hands and falling in love...
A girl that Entropy has already witnessed and fallen in love with. They're not experiencing the timeline in the same order at all!
The goddess indulges in the short time she has before she dies... because she met an Entropy who had already met and fallen in love with the innocent version of her. An Entropy who doesn't share Leylah's childhood, and loves her just as much but in a different way, letting her bask in the illusion of that carefree life she never got to have.
It had nothing to do any physical desire towards the child Leylah. They don't have a normal relationship... but it's because Leylah was never a vulnerable human child.
Leylah was a shadow... shadows don't have their own shapes, they take on the shape of what's around them. The literal center of her soul and self amidst chaos and abyssal darkness was Senadina, who had gone through countless harships. The first who treated her kindly, named her, soothed her fears and self-hatred, and gave her the chance to be a person, was that battered Senadina. Of course she was obsessed and clinging like a child to her. Perception was a very similar kind of being, learning from what she sees around her from a very basic, monstrous, childish base.
And... what we see in adult Leylah was always there, it wasn't caused by how Senadina raised her. Leylah was dangerously obsessed with Sena long before they even had a proper relationship.
Leylah tried to control Sena's actions even before properly introducing herself or having a human appearance, torturing her with a genuine horror movie sequence and hallucinations "for her own good", capturing her at her weakest, even suggesting she kill her friends to survive.
Of course, it wasn't malicious. It was fear. It was always fear. That's been the theme since chapter 1, shadows and fear controlling people, both literally and figuratively. Leylah is the epitome of that, the polar opposite of Senadina's faithful humanity... and before you say she could have been sheltered from the anxiety-inducing horrors better, she always knew what kind of horror Senadina had gone through, Leylah was already, always scared of it, there was no hiding it, she even knew Difeng wasn't real.
The Horrors were just on the backseat while they lived together and Leylah started learning how to be human in the dream. She got to experience life in a society for the first time, got the chance to bond with more people, gain confidence, make friends... she had a long, long road to go before she could become properly self-realized, but she was on that coming of age path, she had the potential to bloom.
The end of that road, the culmination of that potential to be a good person is Entropy. That's the rational, healthy, mostly mature version of Leylah, still flawed, but with enough of a solid foundation to be a person with morals who can coexist with humanity as one of them. It takes a village to raise a child, it took Oxia to raise her into a proper adult person, it took many reassurance and challenges, and she got there. The potential was always there, together with the horrors. It was possible, it was the kind of outcome Senadina hoped for...
Side note but it's important: not only does Senadina protest at the idea of Leylah conforming to her desires in who she grows up to be, going all "that's customization!! no!!", she also states that Entropy is not who she imagined Leylah would grow up into either. It's not because she hoped Leylah would grow up well that she groomed her into someone who fit her tastes. Quite the opposite, the whole point of her relationship with Entropy is that it's a grown up, alternative version of her loved one whom she did not raise, but met as an equal.
As for the elephant in the room.
No, Sena never saw the signs that were there from the start in Leylah.
She admits to Vita that she could have never imagined Leylah would turn out like this... but can you blame her? She incarnates faith itself. Of course she had rose-colored glasses. That's how she kept going through literal hell for hundreds of years and unimaginable hardship. It's a flaw. It's intended. She couldn't see it because that's who she was as a person.
Of course it's a bad thing. Of course she made countless mistakes. Of course she ran herself ragged to the point that it caused even more problems. Of course she was faithful to a toxic point.
Was she a good person? Define good and we'll talk, because hell is paved with good intentions and she could probably find her way around that place with her eyes gouged out.
Her story is a tragedy. She dies with so, so many regrets.
The only way for Senadina to escape her tragic life is to be a completely different girl, divorced from her experiences. A Senadina who is simply not her... The innocent girl that the blood-stained goddess of the red planet never got to be.