Despite my criticisms, this manga is worth the read, if only for the feels, lovable characters, and interesting twists. There no excuse not to; it only has 25 chapters.
Firstly, I say lovable characters, but to be precise, there are three. The hero girl had potential, but all the initial buildup and characterization goes to waste when her personality twists. The same twist happens to the witch, but the characterization until then already solidified, and her suffering makes her more pitable. In other words, the time spent on the hero girl is wasted time.
Secondly, the changing hero-villain dynamic is actually interesting, but not fleshed out enough to provoke thought or emotion. In fact, it's entirely dropped as a theme (besides a few casual mentions) until the end, after I more or less forgot about its existance. It's supposed to be a central idea, but isn't.
Thirdly, commenting on the story as a whole, there's not enough between the twists and feels. The twists and plot are interesting, but come from nowhere with no prior indications of sudden changes, e.g. personality shifts. This might be fine in a several hundred chapter colossus which hasn't planned that far ahead, but not so much in a 25 chapter series.
Fourthly, some characters motivations aren't fleshed out and/or don't make sense. From the protagonist's history, I don't get the impression of someone who wants to destroy the world. He mostly seems like he should be lonely, sad, and regretful - which he is, but for some unknown reason, he also wants to destroy the world (or says so, for no reason, it doesn't contribute to the story much). Also, the entire hero organization basically has no history, they just exist to act as the antagonists and suddenly change their minds so the protagonist doesn't actually have to look for hero girl.
Fifthly, because of that flawed "destroy the world" mentality, the character development branching from it is also flawed. The protagonist jumps between attitudes, sometimes making it seem like he only takes that attitude for other peoples' sake, sometimes making it seem like he genuinely thinks that.
To reiterate, this manga was good. I'm a cold, heartless bastard, so when a manga makes me feel anything beyond the ordinary frustration with cliches or disgust with characters that are supposed to be likable, it's done well.