Well, it's the same thing as when kings are too young to rule and a regent is chosen to do it until the king reaches the age of majority... In an ideal world, rulers would be chosen based on capability and not on birth or just on a popularity contest such as elections.
Well, she did indeed did those bad things. But she got her share of "past stories" with the past queen. And all this that twisted her personality, have something to do with a twisted love.
Later, though there's gonna be a very determining fight, in the end, she will be not as bad as we think she would become.
Her stories and connection with our MC will be a bittersweet one.
I've read and reread ch 1-8, trying to bypass my negative opinions about the brat-face girl, and can say she could have become a pitiable character - IF she didn't say she'll keep killing the prev-Queen's forever.
As a nation without a ruler (due to death or abdication) she was willing to become the next ruler. But since the nation is bent to their "reincarnation object chooses queen" thing, she's pretty much at a disadvantage. There seems to be a set role for women and apparently predetermined women for special roles.
They are essentially telling her, "Thank you for taking care of our nation all these years, but you're not our Queen, we're waiting for her. You'll need to leave that throne when she's back."
In a sense, how long should the nation be waiting/searching/killing for the reincarnate when there's someone semi-qualified.
Just saying, if she didn't trick, threaten, or order people's death, she could have been a pitiable/likable character.