Marcel's methods of punishment, burying other vampires alive in stone, were they starve and desicate.
The reason why Davina is in hiding, her coven holds a ritual that require the murder of their own children for power. The children are led like lambs to the slaughter into a graveyard where they are butchered by their families. When Davina panics and pleas for her mother, all her mother does is look away with an icy stare. Thanks to Marcel, she was saved, but the witches are still looking for her. Gets scarrier when you realize that the witches think they are in the right.
Made even scarrier, in this troper's opinion, when we realize the Harvest actually works, which means the witches have no reason to think they are not in the right.
The curse on Hayley's family is almost the polar opposite of the usual werewolf curse - they are stuck in their wolf form except on days and nights with a full moon.
The fact there is a city where vampires are in charge, where in Mystic Falls vampires are on the fringes of society. In the French Quarter they feed off tourists with impunity, they can hold trial over a witch in public and execute her, leaving her body in the open without fear of reprisal. When the sun goes down, they own the night.
Rebekah's werewolf venom-induced hallucinations while Genevieve has her trapped in an old sanitorium. Visions that plague her include but are not limited to: an old man throwing up blood and glaring menacingly at her, a partially decapitated woman, and twins that might as well be the brothers of those little girls from The Shining that make the most godawful scream. And that's not even the worst of it! The most horrifying being in that hospital for Rebekah is Klaus, after he learns that she'd used Genevieve to summon Mikael to New Orleans to kill him. He stalks her through the hospital, Papa Tunde's knife in hand, roaring her name with fury as she tries to get away...
We finally get to see what happened on the day Mikael came to New Orleans to kill his children. First, he'd physically attacked Elijah and kidnapped Marcel. Then he breaks Klaus in the middle of a crowded opera house by telling him that he will make sure his memory is forgotten long after he's killed the 'abomination' he thinks Klaus is. The crowning moment comes when the show is about to start, and the stage is filled with the impaled and crucified bodies of Marcel and the people Klaus had just made peace with. And then the audience started laughing. Mikael had compelled the human audience to laugh at the horror and think it was just the best thing ever. And after the Originals had fled, he burned down the opera house. With all those people still inside.
And considering what we know about compulsion, who's to say that those people weren't aware of what was going on, but were unable to do anything but laugh while Mikael burned them all?
The hex that the witches placed on Cami's twin brother, and now on her uncle, Paster O'Connell. We get to see the effects of the hex over time. Kieran is aware that his control is gradually slipping away, that one minute he could be fine and the next he could go on a massacre, starting with his own niece. Worst of all, there's no way of reversing the hex once it's been cast.
With the Other Side falling apart, Mikael can affect the land of the living. The first thing he does is torment his sons (and presumably Rebekah, wherever she is) in their dreams. Then he steps it up by almost strangling Hayley to death while she herself is nearly dying from Monique's voodoo attack. And now he's enlisted Davina's help to bring him back to life.
Hayley's screams of horror and fury and helplessness as she's taken by the witches. She's gone into premature labour, and as she's struggling through the pain, she's told that the witches are going to sacrifice her baby to their ancestors.
Just as bad is Klaus' own roar of fury as he searches for Hayley and his child and hears her screaming. When he gets back to full strength, there will be a reckoning.
Esther Mikaelson is still gunning for her children, and it was her decree to kill Hope. It can be argued that her children deserve to die for the horrors they've each wrought since they were turned, but a little newborn baby? Her own granddaughter whose own fate hasn't been set in stone? And now, like Mikael, she's escaped from supernatural purgatory, determined to finish what she started.
It gets worse in the second season: Esther's plan is to undo her children's vampirism by putting their souls in mortal bodies, like she did with Kol and Finn. She wants them to do this voluntarily by - in her own words - systematically destroying everything and everyone that they have ever cared for. And the scary part is she's doing this out of love. Esther loves her children so much that she's going to emotionally, psychologically and then physically rip them apart to 'save' them from the vampirism she put on them. She honest to God believes this.
And she starts with a bang, capturing Elijah and mentally and emotionally tearing him down while he's incapacitated: first with the knowledge that he was the one who killed Tatia, not Esther. And then she traps him inside his mind, in a dream world where Hayley comes to rescue him, and he loses control again.
The fact that Klaus thought it was a good idea to undagger Kol while Marcel was still a child and living under his roof. Elijah's look of abject horror as Klaus merrily shows him the dagger speaks volumes.
Kol using innocent bystanders to re-enact a Shakespeare play, complete with murder and a horrified, hostage teen Marcel as audience . Even worse, he was planning to turn Marcel. As a vampire, all emotions are intensified: what would that have done to a vampire forced to spent eternity as a young teenager, never growing up?
Finn's plan to use untriggered werewolf children in his war against the Originals, by having them murder the human opposition in New Orleans under a blood moon to activate the curse. None of them look older than thirteen.
The introduction of Esther's older sister Dahlia, who used her witchcraft to help Esther conceive...in exchange for the life of Esther's first-born child and the first-born of each subsequent generation that carries her bloodline. If Esther reneges on the deal, Dahlia swears to come back and take all her children.
It gets even worse. Esther is a thousand-year old witch of great power. And yet she's flat-fucking terrified of Dahlia, even more than she is of Mikael, to the extent that she was going to kill her own granddaughter in the season one finale to prevent her from ever returning.
Elijah slaughtering customers at the diner where he meets Rebekah, in a paranoid bid to ensure that no-one knows Hope is alive. Rebekah is so disturbed and horrified by this barbarism from her brother - who would normally compel people first before even thinking of wholesale slaughter - that she snaps his neck to make sure he doesn't lose it near the baby.
Even before this event, Elijah shows signs of losing his control when confronting Kol and Finn. Klaus is the one that has to restrain Elijah, a sure sign that whatever Esther did to him was more serious than originally thought.
Klaus' punishment for Finn? To lock him in a coffin. This, after spending 900 of his 1000+ years as a vampire daggered and locked away. Finn's screams of horror continue to be heard after Klaus and Marcel walk away.
Kol does the same thing to Rebekah but on a more horrible scale. He interferes with Esther's spell so her soul jumps into an unknown woman instead of back in her original body. Said woman is trapped inside the old Fauline mansion, barred by powerful magic from ever leaving.