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.
Venus (Actually Jupiter) already earned the fans ire by attempting to murder Hazel, but she truly crosses into this by attempting it again, this time by stabbing her in the neck (thankfully, it was a decoy), and then she goes on to fantasize about murdering Twilight and her unborn child, possibly not even in that order. Not to mention its heavily implied that she doesn't even actually love Inidouh, she just wants him to suffer for rejecting her. This is proven when she tries to kill Inidouh without an ounce of remorse. Also, unlike Hazel, the author actually saw fit to kill Venus off.
Venus' father, Neptune, probably qualifies as well. Not only was he a crime boss, he was the one who had Venus kill her mother. As the caption for this part said 'when a devil plants a seedů'
We find out later that he tried to force the real Venus to do it, and when she wouldn't, had her Evil Twin, Jupiter (the Venus we know) kill her while Venus was Forced To Watch. When Venus wouldn't stop crying, he took her to Zebrica and left her for dead!
Fridge Horror: Nova regained Astra's memories of Jupiter. Not only does this include multiple murders Astra knows of, it includes strangling their mother to death. And Nova feels guilty for it. Poor mare.
If not then, then by trying to kill Iniduoh later on after he offers her a Last-Second Chance, proving any feelings she may have had for him are long dead.
Neptune crossed it by trying to get the real Venus (who would become known as Astra) to murder her own mother, then when she refused has Jupiter (her Evil Twin and the Venus we know) do the deed while Venus was Forced To Watch. This probably also counts as Jupiter's Moral Event Horizon as well. As if that weren't enough, when Venus quite reasonably wouldn't stop crying, he flew her to Zebrica and left her for dead!
How she killed her mother, especially how Venus describes it. Imagine being tied up and forced to watch your twin sister strangle your mother to death while the poor mare begs her to stop!
And as we find out later, she kept the rope as a memento!
Hell, the box in general. Jupiter kept mementos of her worst crimes in that box, and went to great lengths to ensure only she would ever be able to see them (Mars finding the key thwarts this), which suggests she treasures them. Her most treasured possessions are memoirs of all the pain she caused.
The short story arc involving Jupiter being brought Back from the Dead (or, more accurately, cloned). The clone Jupiter may have been more Ax Crazy than the original, but she was still terrifying, as was her return.
The Woobie: Astra ,AKA the real Venus, sweet Celestia, Astra! Her father and twin sister were Complete Monsters and the only one who was genuinely kind to her, her mother, was murdered while she was Forced To Watch after her father tried to convince her to kill her mother herself. Then, when she (quite reasonably) didn't stop crying, Neptune left her for dead in Zebrica. Is it any wonder the poor thing is a nervous wreck?
Then she tries to bring her dead mother back to life using a magic stone she got from the Jerkass GenieBig Bad of another blog because of how much she missed her. The stone ends up bringing Jupiter back instead.
The Jupiter Clone, after her memory is wiped by the backfiring Serpent's Eye. The first time we see her awake is her sitting lonely and confused in a padded room, not even remembering how to speak. On top of this, there's a possibility she'll be killed by the authorities due to how she was born.
And thenshe gets her memories back. When Nyx talks to her later about it, Nova reveals she has Jupiter's memories (or more exactly, Astra's memories of Jupiter made up like hers, which is still horrible), but unlike Jupiter doesn't have a Lack of Empathy and thus feels the guilt for things Jupiter did but she never did.