These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Designated Villain: Rumpelstiltskin. Yeah, asking for the baby was rather sinister, but up until the final act he really is not evil. It wasn't until the protagonist choose not to up hold her end of the bargain that he became an antagonist.
Family-Unfriendly Aesop: The girl refuses to keep her end of the deal and only wins against Rumpelstiltskin by spying on him. Rumpelstiltksin holds up his end of the deal and gives her a second chance out of the kindness of his heart, then dies a horrible death. The moral of the story? Cry and cheat to get your way, apparently.
Then again he demanded her child as payment for helping her. God knows what he wanted to do to the kid.
A common interpretation based on his song is that he planned to eat it. However, while an easy conclusion to leap to, this isn't actually stated in the verse.
Considering that he was likely some kind of fae, it's entirely possible he wanted the child to, well... have a child. In many stories about fae they don't so much have baby fae as take human children and turn them fae.
Regardless of what his intentions were, she accepted the terms of his deal. She was, at least for a time, willing to throw her firstborn under a bus to save her skin for at least another night.
Though she accepted the terms of the deal under duress; if she didn't, the king would have put her to death. The agreement was about as valid as a contract signed at gunpoint.
The miller's daughter is threatened with death and ordered to complete an impossible task. If she reveals that her father was telling tall tales, he'll probably be killed. After three nights of this psychological torture, she is forced to marry and bear children for the king, the man who kidnapped her, threatened her with death, and locked her up for days. She then spends the rest of her life petrified that the king will find out that she and her father are liars. (Lying to the king is an act of treason, and the punishments for treason were pretty nasty back in the day.) With Rumpelstiltskin dead, what happens when the king's treasury is low and he asks his wife to spin more straw into gold?
Jerkass: The miller's daughter. She gets told the whole story about how the guy found Rumpelstiltskin's house and eavesdropped. Instead of just announcing the fact, she found out his name, she starts her sentence with "Heißt du vielleicht Hinz? Heißt du vielleicht Kunz?" Literally: "Maybe your name is Hinz? Maybe your name is Kunz?" Which were both terribly common names around the time(to the point "Hinz und Kunz" has become a proverbial way to say "literally anyone") and as it was her final chance to guess his name, she probably already guessed them the first time around. Only after dropping these two names to raise Rumpelstiltskin's mood, she breaks him with "Heißt du vielleicht Rumpelstilzchen?"