Twilight qualifies big time... At least during Severing and most of Reconnection. Then came Breaking the Circle, and it seems that Twilight herself grows resented with it all and works to push herself through a Moral White Hole, complete with erasing all her memories of the repeated alternate universes when the vicious cycle is finally stopped.
Celestia (Except for the one in the last timeline in Breaking the Circle) is not exactly a saint either, being The Mare Behind Twilight and the mastermind of a Super Soldier program to get a paragon of magic that could be able to sever the Elements and tilt the balance in her battle against Luna. The epilogue hints that this program has left thousands of foals and fillies orphaned or separated from their parents forever for the sake of obtaining the perfect unicorn magician.
For her first betrayal, Twilight shoots Dash down with magical lightning, completely severing one of her wings while she's flying through a storm.
Her second act involves her falsely promising Applejack that she'll ensure both she and her daughter survive birth. When Applejack's death doesn't completely sever her element, she kills the newborn without a second thought.
Her third plot entails telling Pinkie Pie exactly what she's got planned, solely to ensure that she's properly horrified when she blows up the Sweet Shop with Pinkie, her children and Ms. Cake inside. To top it off, everyone within a two block radius is killed.
Her fourth betrayal sees her not only destroying Rarity's career through embezzlement, but burning down the boutique while she's still inside. When Rarity survives, she visits just to explain all the ways she's wronged her and laugh in her face.
Her fifth and final plan was simply to make Fluttershy watch as she slaughtered all of her animal friends with her magic, one by one, until the sheer cruelty of it all overwhelmed her.
Ron the Death Eater: Twilight all the way. Celestia gets plenty of this, too, being the one who set Twilight up to it.