Its not the most complex or "deep" motivation but its simply this: MC worked her ass off, fought for and improved the quality of life for EVERYONE in the kingdom as that was her only motivation while there: bring happiness and prosperity to the people; but because of one baseless claim they cast her aside, turned their back on her and decided to label her a traitor - even her father and the King know that this didnt make sense but instead of helping/fighting for her, they also cast her aside - and even the villagers turned on her immediately
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So she decided that she'll turn on them too. Thats the whole story. the thing we have seen and has been mentioned multiple times is that the kingdom isn't doing well without her and the prince keeps making things worse even without her input. She simply wants to show people just how much she did for them and make them sorry they ever turned their backs on her. ╮( ̄▽ ̄)╭
Bro she lives in the era where manipulating the masses is as easy as manipulating cattle, the masses are ignorant of the true political quarrel in the government, but sure lets kill off innocent children that she onced care for, wanted to bring happiness to in the process of taking revenge. It's like wanting to cut your classmate's finger off because they believed in a false rumor that the teacher spread. It's not complex or deep but it sure sounds childish as hell from what you are saying. But yeah sure i guess there can be charm in a... childish, vengeful noble lady, however this lady action might be childish and vengeful, her usual demeanor expressed nothing of the sort, just a cookie cutter noble lady, whose is all kind and nice but her revenge action left the viewer blindsided because the plot is embarassed of itself and doesnt even want to properl portray the direction it is going for
Fair enough. I dont believe its well portrayed either. But I think a lot of people get caught up in the aspect of her personality being one way yet her ultimate actions demonstrating another. Many people bring up the killing of children part, which obviously was terrible and I'm not some kind of sociopath to no appreciate that, but I struggle to see how they expected her to get the revenge she's wanting without harming any children at all... political games always have repercussions on those bellow. And most of the time its in the most callous of ways (check current events for evidence of it)
Again I'm not saying her personality or the motives the author has displayed have been executed well, I'm just struggling to see why this can't be a silly indulgent read like many other stories of this genre

Somehow this story tell a lot but barely show. I mean i guess she got betrayed and used but there's only the maid and the mc info dump through words.. like yeah it's realistic but we didnt get to see what's going on? Other character's perspective..? They did discussed about OTHER people discussing rumors but that's about as far as the set up goes, at least other slop revenge showed us exactly the motives or the opinion of the pepertrators so we can get angry.. but this one? Barely afford to describe a proper scene it's actually sad..
The mc barely loose any composure ever so we cant actually see how human she is. Why did she spared that one threat to her bussiness but cursed the knight, hell, the part where they talked for the first time after a while we didnt event get a glimpse into whether or not she actually holds a grudge or not, and when we get to the part where she said she didnt forgive him i was like "oh ok? We're killing him? I did not see that coming i guess? Oh? We used him to kill some children too?oh Ok i guess??"
I guess merciless mc can cherry pick who she want to kill and who doesnt I guess? Then i go back to chap 1 to see if he did anything THAT AWFUL to her i only get to see that guy in barely 1 panel like bruh, her revenge is so cheap lmao. Ntm the spell she casted on him, the one that she used to secondhandedly kill innocent children, that spell's drawback is her only getting her hands hurty for some days?? Gimme a break lol, the manga made it so the maid looks concerned about the price of casting the spell only for it to be so cheap it's physically hurts. Oh and the knight? Can we at least get a glimpse on how traumatizing it is for him after the massacr- NAH BRO, best we can do is some OTHER dude mentioning HIM mentioning how he didnt do it and have some ptsd or sth, maybe give him like a 3 seconds of clearance before his death to fearmonger the people about the ooooooo scary MC oooooooo.
I dont even want to talk about the MC half ass moral lmao because even her motives is barely potrayed.