Daemon from the Tamers Forever Series could be the posterboy for this trope, a nigh unstoppable psychopath who is fuelled by his own hatred, often exploding into violent rages and laughing maniacally at the sight of others in pain.
Joe Dark from Clash of the Elements is a very prime example of this trope and this very trait of his is amplified immensely after he gains the Dark King's powers.
Similarly, in Inner Demons, after Twilight is consumed by her Queen persona, she becomes an extremely mentally unstable sociopath who switches between cold emotional detachment and blind fury at the drop of a hat. It gets to the point where she even decides to kill her brother when he rejects her advances.
And that's before her Villainous Breakdown. Afterwards, she's left constantly switching between laughing maniacally and sobbing (it's actually implied that's her true personality trying to resurface and being forced back down by the Queen), and on top of that she's decided that rather than ruling Equestria, she'll destroy it.
Nightmare Phobia, the Big Bad of the Chaos Verse (an official spinoff of the POV Series above) is this trope in spades. And it's not helped at all by the fact that the more powerful she becomes, the more her memory is corrupted, which in turn only fuels her insanity more.
A Growing Affection has the self-named Reaper of Shinobi. She thinks that anyone who uses chakra (herself included) is a demon, so her goal is to kill all ninjas and then kill herself. Over the years she has come to believe the backstory she fabricated for herself (that she is the sole survivor of the destruction of her village in the last war).
Several characters in You Got HaruhiRolled!. The fic follows canon in portraying Ryouko in this way, but in addition, Haruhi is occasionally Flanderized into this trope, such as when she threatens to kill Kyon's sister for dishonoring the SOS Brigade. Additionally, Sasaki usually functions as the Only Sane Woman of her group, but in the chapter where she finds a Death Note, she arguably out-crazies YagamiLight himself.note Though this is only implied, rather than outright stated.
Twilight Sparkle is the prime example in Pages of Harmony, though Rainbow Dash is definitely no slouch in that regard either. Twilight basically tortures and Mind Rapes her friends to extract their Elements, becoming increasingly deranged the further she goes down the road. Rainbow Dash, the first killed as a result of Twilight's actions, has her memory twisted and warped to the point where she cannibalizes herself as a result of no longer having Loyalty to even herself, and swears that she'll find a way to kill Twilight for what she did.