Why is Princess Reiners called a monster? We got introduced to Princess Reiners in season 2, episode 1, and we were given flashes in the ending theme that she had a dark personality. This was later confirmed in episodes 9 and 10 when her brother, Prince Zanac, calls her a monster to her face. When she turned around, she had the anime crazy face, but didn't actually change form into a literal monster of any kind.

Over on the MAL forum for episode 12, they simply refer to her as a yandere. On the Wikia, they say she's a psychopath.

I always considered the anime crazy face to be a metaphor for viewers, not a literal change for other characters in-story. Aside from her dark personality / face twisting, and the fact that she plans out big, complicated, moves, I don't understand what the big deal is. Why would she be called a monster? How would anyone else in Yggdrasil would know that she is a "monster"? Or is she a monster in a literal sense?
Princess Renner belongs to the Human Race and doesn't possess any magical skills or powers such as other races. So the people referring to her as a Monster is just symbolic or figurative.
For the reasons why she is called a monster, it's mainly because she hides her true twisted personality and acts just the opposite way in public.
Aside from this, she's extremely observational, intelligent, and possess advanced intellect.
There is a time when she easily deciphers various locations of eight fingers from a document, and then we see Lakyus (leader of Blue Roses) saying that there is no one for whom the word genius is a better fit except the Princess.
Then during a meeting with Marquis Raeven and her brother Zanac, she says some interesting dialogs about Marquis

I wish for you, the hidden leader of the Royalty Faction, no, the one
who controls the Royalty Faction from the shadows, to lend me your
household troops..........

Both Marquis Raeven and her brother Zenac are terrified and Zanac says:

How do you, up here like a bird in cage, know all of that?

That is the reason why Zanac describes Renner as an unfathomable monster.
Some more quotes:

Why, Climb. Are you on your way to show your face to that monster?
To Climb warning about Renner: Listen, Climb. If you were a bigot then I wouldn’t have even bothered
to say anything. But I’m giving you a warning since she could be
tricking you. She’s a monster
To Renner: Is that so. This is your true face. What should I say when you were
young, it always felt like something was strange about you, but now I
know you’re not normal


So considering these characteristics, it can be said that she is the most intelligent human in the whole Overlord series and also gets on well with other superior races (for example, check her relations with Albedo in the other question regarding Renner). It's just natural for anyone to be terrified after learning her identity behind the mask she puts on as a saint. You would really call someone like that a true monster.
So in short, she is not a (former) monster in any literal sense. But her personality, intelligence, and mental disorder makes her one (symbolically)