Honestly, the Roam family isn't irredeemable.
BEFORE YOU SAY ANYTHING, I DIDN'T SAY THAT THEY SHOULD BE FORGIVEN. I'M JUST SAYING THEY'RE NOT PURE EVIL.
Lilia was churning out "prophesies" (read: novel spoilers) that were all coming true and predicted that Annelie would bring the downfall of their family.
Between the terminally ill daughter who was "prophesied" to bring the family to ruin, and the "prophet" who was ready to fill in her shoes, it was obvious which choice was the "correct" one.
The family has shown regret, snippet below.
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"The prophecy said that Annelie would bring ruin to our family. But that hasn't happened."
"Because we put her on the execution block first."
Duke Roam's lips trembled.
If they had all been deceived by a false prophecy... The thought alone was terrifying.
"Before she could destroy the house of Roam... we killed her first. And now there's no way to verify that prophecy."
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The family seems to think that Annelie would be safer with them than with the temple or the palace (MAYBE true) and that Annelie would understand their decision if they explained themselves.
Right now their mindset is "no use regretting or lamenting (they're still kinda doing this btw), just look ahead make the best decision for the future"