Why would you want someone to become a tyrant and kill a bunch of people why would you wan...

She said it was to go back to her own world. Ngl if i was sent into a novel and the only way out was to do all that, I’d do it.

Because that her role. She isn’t from that world and the only way for her to go back is to continue the novel how it originally went. And in the novel the ml killed his father (the emperor) and she was framed for his death (basically sacrificed herself to save him so she can go back to her world) when she died he basically lost himself to madness and became a tyrant. The only way she can return to her world is to make the ml a tyrant which is what she is trying to do.

Welp... if you have a good life and think of this as inside the novel and your world as the real world instead of current life and previous life and want to return to your own world... and if you know he will be happy after all that... well... I'd do the same...
Even more so if she is from a modern world. No matter how rich or luxurious your life may be in that setting. If you consider the development of the modern era that luxurious life, like food and comfort woud be worse than a mid class modern life. From the uncomfortable clothes, buildings quality, furniture, hygiene necessities... no tv, no phone, computer, internet... not even electricity... no liberty to go wherever you want, learn whatever you want nor marry whomever you want... it's depicted beautifully on comics and novels... but realistically speaking... just how unhappy you would have to be in modern world to not want to return to it if you got the chance?
So yeah... I'd choose the same... rather than selfish... that would be the normal reaction? If I was in her place and went back in time I'd probably go crazy and become a tyrant myself... and very likely I'd torture the fricker responsible for stopping me from going back home to death... no matter what he might feel for me. ╮(￣▽￣)╭

See that mind set hinges on the idea that not only are these people who you have spent literal years with not real but also that their suffering holds no value beyond getting her what she wants thats such a disturbing thought I'd believe it if she was younger but she's 37 I'd believe it if she spent a couple of days maybe even weeks with these people buy she grew up with them its been at least a decade and she still doesn't believe that this boy's who is supposed to be a childhood best friend who she spent a bunch of time with suffering and the suffering of innocent people who would be killed under his reign unnecessarily just doesn't matter like that's so incredibly short sighted and cold hearted what the hell

Because to her it is. It was made clear to her when she was sent there that it was the world inside a novel. That she had to play a role there if she ever wanted to see her family and everyone she loved again. Then what? Should she give up on her life and her world over a childhood friend she didn't want and strangers? You say is short sighted and cold hearted I say is hypocritical to expect someone to sacrifice themselves like that over people she know are characters in a book. No matter how much you say you'd do differently you would not. You forget that she is not there out of free will. She was abducted and forced to play a role so she could go home. If she wasn't told she had to do that in order to go home she would do her best to not be involved with people from there and try whatever she could to go home.
Yeah she get swayed. But she steeled herself and did it all the 1st time cuz she believed he would get his happy ending eventually. And this time she will be swayed till she falls for him deep enough to forgive him before finding out he made them go to past out of love for her. Cuz if she found out soon after going back any affection she could have for him woud turn into hate.
And you are considering it as it being a true world and she being selfish for not minding others suffering so she can go home. Then how about you think the contrary? How selfish of the ML to turn back time and kidnap her soul and bind her to his side instead of letting she go back to her life? What anout her family and friends? Are they not suffering from having her taken away from them? Cuz he didn't do it cuz a lot of people died but cuz he couldn't get over the loss of one person he holds dear. So. How many dear people lost her and she lost in order to fulfill his happiness?
Honestly, no one (who has a good life) would be willing to sacrifice their life in the real world to be able to get inside a book to stop the suffering and death of fictional characters and that's what you are asking from the mc. She was sent inside a novel against her will and had to complete her role to go back home. Can you even imagine how she would feel having to interact with Biche's family while being forcefully separated from her own? Ot with him while thinking that if he didn't exist she wouldn't be in that situation? How many times she must have cursed that book, the author and whatever sent her there? Why do you put the weight of everything on her shoulders? Why should she be responsible for changing it when it would happen anyway if it wasn't her soul there?
This is from the point that was told to her. That this is a fictional world and that all that suffering must happen like in the novel and that if she ever wanted to go back home she had to play that role. Not the assumption this is a parallel word and that everything is real. She doesn't know it the same way he doesn't know she is from another world and would went back to her normal life after dying.