Personally, at least to me, while the information in the original Visual Novel doesn’t really excuse Bernkastel in the traditional moral sense, it very much humanizes her actions to a degree if you look closely. Both what Umineko states about her being trapped in a logic error akin to a monkey at a typewriter blindly smashing at the keys and writing a masterpiece, as well as (Higurashi Spoilers) The details of how she went through loop after loop in higurashi of watching her friends go mad and kill each other and ending up dying in a rather brutal way in every loop except for the last/ Not only that, but the personality that is Frederica Bernkastel never got to enjoy the spoils of victory as she left the body of Rika after Saikoroshi-hen, which meant she did not even get the opportunity to enjoy the hard work that she put in to creating a miracle.
After she was freed from the Logic Error, she was probably quite distraught and damaged, and when Featherine found her, she ‘taught her the taste of meat’ as it was stated in the end of Episode 6. That is, she was taught very much something similar to what she had taught Erika, she was taught to cause others suffering to deal with the very real issues she herself had from the Logic Error that was Featherine’s fault in the first place. And we see that Bernkastel resents Featherine for it, and even fears her, which given the normal personality we see from Bernkastel, says volumes about Featherine as a character off-screen.
Personally, because of Bernkastel’s personality, I would say the very fact that she puts up with half of what Lambdadelta does to her, especially in some of the TIPS, such as Whose Tea Party from Tsubasa as well as the ??? from EP 8 indicate that Bernkastel really does deeply care for Lambdadelta in her own way. Witches are alien creatures who would be very well comparable to the Faeries of old, so while their perceptions about something such as love would be probably extremely different than Humans and be quite twisted in our perspective, I do not doubt that whatever passes for love among witches is what Bern and Lambda have, and a very deep form of it as well.
Another thing of note is the resting trauma eyes from Bern, and while they may simply be a stylistic choice, I do not think that is the case. With how much trauma she is said to have gone through in the time leading up to the present, it would be a small wonder if she wasn’t deeply traumatized by both the events of the logic error as well as whatever horrific training that Featherine put her through. I shall reiterate that what she did was definitely wrong on the scale of humans, but I would argue that on a deeper inspection it at least becomes understandable, if not downright sympathetic like someone along the likes of the Greek Heroine Medea, or (Higurashi Spoilers) Higurashi’s own Miyo Takano.
As for her role in the events of Chiru, I would argue they make coherent sense even before the retcon without just attributing it to her being the villain of the story. It was rather stated that she had a tendency to be an absolutely sore loser, and we could see that in how she treated Erika. So it is no small wonder that when Featherine gave her the opportunity to tear apart Beatrice’s game and torment Battler and Beatrice, who had squarely handed her a loss, she would jump on the opportunity and take it to avenge the humiliation she was given.
On the topic of boredom, it is a very strong motivator for Voyagers in general, as it literally can kill them, but especially for Bernkastel. She, and probably many other voyagers, constantly seek entertainment as a sort of nepenthe to escape from the horrors of her past, and the cruelty is just how she knows to do it best, as that is what she was taught by Featherine. Which we know she resents, so perhaps deep down, she hates what she is doing but does it anyway as she cannot bear to face the horrors of her past and indulges in the vile ways she had been taught by Featherine much like an addict indulges in their drug of choice.
And perhaps this is another point where her love for Lambdadelta comes in. Perhaps being around someone like her gives her entertainment and a way to forget about her past without indulging in those terrible habits of hers as much, and allows her to be more who she is. Or perhaps having a clear enemy and polarizing the fight contributes to her enjoyment as well which makes her less in need of cruelty to run away from her past. An alternate way of coping that keeps her from getting worse than she already is.
Or I could be completely off-base and just letting my own biases shape how the character appears to me, and am looking way too deeply into it. I’m interested to hear what others think of my take on Bern, so feel free to tear it apart, I’m tough enough to handle anything really.