Honestly, the translation is kinda confusing, so I'm not sure which of us misunderstood.
But from what I understand, it's not that she didn't want to help, she just didn't know the right way to save him.
The prince seemed to be hated by the people in the palace, so he was lonely and his heart was empty. She assumed that the role of heroine in the game was to fill that hole. But she never got to play his route, so she didn't know how. Not to mention, she's the villainess. And given how she felt she couldn't help her siblings in her past life, she felt even more unsure about how to save the prince.
But she knew it would suck if the prince grew up alone. So, she wanted to become his friend. She wasn't planning to be someone who can save his heart, but she would become someone he can talk with. At least until the prince meet the heroine.
So yeah, I don't think she's a bad person.
These are all great points, and I don't really disagree, that just isn't how it came across to me; maybe it
is the translation, as you suggest.
To me, it seems like the MC's initial reaction is one of mild disgust and apathy; some bother that she just can't stand, but may as well deal with for a second... and then she justifies being nice to him while giving herself a somewhat negative pep-talk, and decides that she will not actively treat him like shit... as long as it's convenient.
That's how it reads to me, even a second time through; she doesn't give a shit about a
borderline abused child because it would take minimal effort to say something nice, and minimal effort is almost too much.
And that is a thoroughly shite human.
A person who doesn't have enough basic compassion to comfort a small child in pain? That's plainly somebody the world is better off without.
But maybe it is a poor translation, and she isn't doing that... but I have seen the exact attitude in about 576 different manga, and it's a fairly normal way of thinking for some places in the world, so... not sure it's a mistake.
Remember: in most manga, helping other people is a "selfish" act.
And, if helping is socially negative, then this translation is spot on... and she is a shite person (by western standards).
....
I think I'd rather be wrong than have this all be real... but I'm not convinced I am...
[EDIT]
I forgot, and now hasten, to add:
She does any like a wonderfully compassionate person in the very next chapter; but the "getting there" part of it, and the rationale behind it, is simply odious.
Unless, as stipulated, it's just a wonky translation.