Lighthearted show though it was, The Worst Witch had an episode called "The Genius Of The Lamp" where Sybil and Clarice create a magic lamp and wish for unlimited wishes. The catch is, whenever Sybil makes a wish, the lamp draws energy from something else. When it runs out of non-living things to drain energy from...
The sheer fact that potions like "personality change," forgetting powder, and such are easy to make, and taught to young people. There's a lot of nasty stuff you could do with them if you had malicious intent...
Merging with Paranoia Fuel, the Uninvited from the Season 3 finale. She puts the whole school into an enchanted sleep thanks to the girls summoning her by accident from the Tome of Eldritch Lore. Her actress is also quite sinister.
The series makes Mildred being turned into a frog even scarier than the book - where she just woke up in that condition. Here, we see someone face unseen sneaking into her room to leave weeds on her pillow and then doing the spell. Fridge Horror when you realise it's lucky Ethel isn't pure evil - as with her knowledge she could likely do much worse.
In the series, Agatha and her witches actually manage to sneak into the school invisible and turn Miss Cackle into a frog. It's quite startling when they surround Maud and start cackling.
Ethel and Drucilla pushing the boat out in the series is likewise made even more shocking when the episode ends on a literal Cliffhanger - the river approaching some dangerous rapids. And this time the girls know Tabby is on board!
The 2017 series
Agatha Cackle, full stop. She does a perfect impression of the heroic Ada Cackle, knows some very powerful magic, and nearly succeeded in taking over Cackle's Academy, twice!. At least it can be stated that she was willing to show some mercy to Ada, considering they were sisters after all, so Agatha's not completely heartless.
Out of Bounds/The Worst Headmistress:
Agatha tricking Esmeralda into giving her her powers (Esmeralda thought the lady was Ada and that Agatha stole her powers). By the time the real Miss Cackle walked in and realized what happened, it was too late, Agatha realized the school was rightfully hers, and took over
Agatha and Gullet's rampage in general can be considered this. Anyone the duo dislikes will be transformed by one of them, no unauthorized spells could be made without them finding out, Ada Cackle was trapped in her own painting, and Mildred was out of the action for the majority of the episode. Even the Great wizard couldn't handle their power. Plus neither of them show any remorse for their actions, or even try to back hold back for harming the pupils. Sweet dreams, viewers.
The idea that people (let alone children) can be turned into cakes and eaten is disturbing to say the least. It's a good thing Agatha and Miss Gullet were stopped when they were, or we would have been short three central characters. (Maud, Enid and Ethel)
Agatha nearly destroying the academy when the staff and pupils try to rebel against her, luckily the others were able to fix it, and take down Agatha.
The fourth season introduces Wormwood Academy, a school created for the 'worst witches' in the sense of those who committed some dire offence, such as one student having accidentally turned her sister into a cactus. The head of this academy, Miss Splinter, spends every moment she is interacting with her 'pupils' constantly making it clear to them all that she doesn't even consider them to be true witches, telling them that they don't deserve and can never expect to be anything more than horrible failures.
