This might sound a bit out of context since I was replying to another person's reply in that debate but here goes:
That's because they didn't even know that he was suffering at the time. From what I remember Helios probably did try to visit him in the original timeline too but just like how it is in this one Cael shut him down, and unlike now, there was no Hestia to help them reengage. In my opinion, although they had flaws and wrongdoings, they're forgivable. Taking revenge against them to a certain extent is okay, but in the novel where it went more on detail about what Hestia wanted to do, she already went a little over that line. They weren't consciously hurting Cael in the first place, and it wasn't as if they killed him with their own hands. They never wanted to hurt Cael, they just accidentally did so while following their beliefs/feelings and didn't notice it, which is a fault, but not bad enough to warrant complete destruction. Even after they apologized she still wanted to harm them more. She was even mad at Cael at some point because he defended the crown princess from being deposed. Not only that but she also realized in the novel that the revenge she was doing was not entirely for Cael. She was also trying to take revenge on them relentlessly because without it she believed that she would have no other 'purpose' in her life, something which Cael worked hard to fix.
I have the opposite opinion. I started disliking Hestia because even the victim already thought it was enough and yet she still kept pushing for something he no longer wanted. The way I see it, she doesn't view them as humans but rather characters in a novel still. She doesn't empathize with what would happen next to them if she kept unnecessarily pushing them into the corner. It's true that they indirectly caused Cael's death in the past life but they never wanted to actually kill him. Helios got Diana fair and square, and Caelus got punished for assassinating a noble family without trial (although it was for the benefit of the empire). She might look badass but in a way, shes also flawed. She's not the perfect revenge FL you make her out to be. If you look into that one line she said in the novel and think about it deeper you can almost faintly tell that she herself knows in the back of her mind that for the most part, aside from Cael's death, she's acting upon this revenge for herself because it's her 'only purpose'. She never thinks that it's enough, if Caelus didn't stop her, if the situation permitted it or if the author didn't want her to look to obviously unfeeling I feel like she would've even wanted to drive Diana and Helios to death. One thing that starts missing once people start gobbling up the actions the cool FL does is the morality behind her actions. Sometimes it goes beyond the line of 'cool' towards the area of 'unreasonable'.
Other stuff I copy pasted from a previous debate I had:
Yes, I did mention that he did it for the betterment of the country myself in my previous comment. HOWEVER, he ended the whole family line without an official trial. If we're talking public law, he basically just murdered/assassinated a whole family of nobles. That being said, there was no way Cael would've gone unpunished, mostly due to other noble families' backlash. They would've said that the royal family and Cael were in collusion/corrupt/biased. With that in mind, it was Helios' duty to give Cael punishment as the crowned prince even if they were friends. Hell Cael should've gone through the death penalty but he didn't because of his contributions to the country and their relationship.
What I learned from reading the story is that most of the characters are not just black and white:
Helios punished his friend and married the person his best friend also loved but he didn't do it with the intention of hurting him. He fought for Diana fair and square, and won. He also believed that since he 'got her' he should show Cael that they were happy so that he'd be rest assured that the person he loves was being treasured and protected by the one who got her.
Diana lived her whole life in church and never experienced hardships. Her whole world was the church, Helios and Cael. It was inevitable that she would grow extremely attached, dependent, and possessive of them. They were all she ever knew. Unfortunately the dark side to that is she was never able to properly process her feelings due to her overprotected upbringing so that attachment festered into regretful infidel love.
I'm just waiting for this to end at this point bc I'm already so done with Hestia after reading the novel like a year ago. After reading the novel I can't see her as charming as she used to be at the beginning.