I agree with others up top. Like what was listed by TIAB:
Black Fairy - continuously kidnapping and torturing children, working them to death in the process, lying alot, attempting murder, tried to destroy every realm.
Blind Witch (s1) - eating children.
Captain Hook - willing to risk others lives just to have his revenge on Rumple or Emma for what she did to him, willing to have everyone sent in the underworld, breaking verbal deals with Ariel and Ursula II, selling out Bae to Malcolm, ripping out Aurora's heart out, murdering fathers of Children (especially Charming's, which still needs an ending), double crossing, lying a lot, leaving Charming under a lie about his dad and still did it in the present, and leaving Bae and Rumple under a lie about Milah's so called death (which she was involved in too).
Cruella De Vil - Controlling Animals, murdering her fathers and mother, breaking and entering Snowing's castle to force them to help her in a threatening way, born evil, lying alot like she did Maleficent about Lily and to Charming about why James has a grudge on him.
Dr. Facilier - bringing terrible prices to others like Naveen, kidnapping Robert the Frog's love, murdering Hansel II.
Evil Queen/Regina - murdering innocents, child abuse, lying alot, breaking verbal deals, orphan making, murdering Snow White's father, murdering Graham (which still needs an ending), raping Graham, Double Crossing on people like Jefferson, Sleep cursing Snow, helping Maleficent sleep curse Snow, murdering Percival's family, cutting Lily's hand for blood ink and not showing regrets for it, murdering Wish Henry's grandparents, framing cursed Snow for murder, ripping children's hearts out like on a lost boy, casting the dark curse to hurt Snow, trying to murder Emma when she was a baby, robbing the life Snow was meant to have with her daughter, involved sleep cursing Aurora, ripping people hearts out and still left them in boxes, sending people like the count or Huntsman to assasinate the Charmings.
Gaston - double crossing, framing and trying to kill a young ogre, lying alot.
Greg & Tamara - almost murdering everyone in Storybrooke, attempting to destroy magic, almost killed Bae and Pinocchio, kidnapping Henry, lying alot.
Hades - murdering Robin Hood, lying alot, sending innocent souls in the river styx, attempted murder, attempting to conquer the world with the lightning bolt.
Isaac Heller - Orphan making, making others do things against their will by the writing of his pen to alter their lives, lying alot, attempting to get others killed, trapping everyone in his book and alter their lives (before Henry erased everything).
Jaqueline - double crossing Anton and murdering his brothers
Jafar - trapping Agrabah in a little crystal ball, breaking the rules of magic to become more powerful, murdering people like Anastasia (before her resurrection)
King George - murdering Charming's mother, trying to come between the Charmings, lying alot, cursing Snow to not bear a child, murdering innocents, framing Red for murder, blackmailing, forcing and threatening Charming to do what he wants him to do.
Lady Tremaine (s6) - almost murdered Cinderella, abusive to her and her daughter, forcing her girls to do what she wants, lying alot, blackmailing.
Malcolm - kidnapping Henry or other children, murdering, lying alot, abandining his son in exchange to regain his youth.
Maleficent - sleep cursing Briar Rose (which this story is left untold and unfinished), sleep cursing Aurora (bith still left as enemies and story of feud still left unfinished), breaking and entering Snowing's castle to force them to help her in a threatening way, turning Phillip into a monster, coming between Aurora and Phillip, being involved of sleep cursing Snow too.
Mordred - trying to usurp King Arthur's throne.
Mother Gothel - abandining her new daughter, conquering the world, destroy humanity with mass genocide, murder, orphan making, willing to turn Alice into a plant, raping Wish Hook and cursing his heart to keep him and Alice away from each other.
Mr. Hyde - being spiteful to others, attempting murder, immune to magic being used against him.
Nimue - wishes to rise from the dead with all the dark ones and have victoms take their place in the underworld, betrayed Merlin.
Prince James - trying to murder his brother cause he claims he stole his glory, double crossing Anton and murdering his brothers.
Queen of Hearts/Cora - Lying alot, murdering Snow's mother, murdered innocent people, force Regina to do what she wants just to take Leopold's kingdom, framing Regina for murder, murdering Daniel, lied about Lancelot's death, trying to steal from King Leopold, earn more power and become the new Dark One, ripping people's hearts out, trying to rip Snow's heart out as a gift for Regina.
Rumplestiltskin - murder on innocents, manipulation, plan a curse to find his son (which is partially a good thing), lying alot, trickery, bringing a terrible price to others, getting someone else to do a crime of his.
Snow Queen/Ingrid - trying to force Emma and Elsa to be her sisters, shattered cursed everyone to turn on and kill each other, lying alot, deceiving herself alot when someone like Anna speaks the truth.
Ursula II - breaking and entering Snowing's castle to force them to help her in a threatening way, attempted murder, terrorising the seas.
Wicked Witch of the West/Zelena - kidnapping baby Neal for her spell just after he was born, raping Robin Hood, turning people into flying monkeys, black mailing Aurora and Phillip, murdering Baelfire (which still needs an ending), leaving Hansel & Gretel to get eating by the blind witch, lying alot, sleep cursing Dorothy, trying to kill Dorothy because of her jealousy of her (which that feud story is still left unfinished), betraying the Tin Man.