NMM and Sombra have open back stories.
Even Babs had a back story explaining her actions. Sunset Shimmer too, and hers was REALLY weak so she's on my hate list too.
Diamond Tiara's back story is that she just enjoys bullying other ponies. She has nothing redeeming about her at all. Villains aren't born, they're made. Usually something TERRIBLE happens that turns a character into a monster, DT has EVERYTHING going for her, with no agenda other than to hurt others because she thinks its fun.
She wears a freakin' tiara on her head. Yeah she needs to be taken out back and have the ever living sh** kicked out of her. In the Season 4 previews she makes fun of handicapped kids, she needs to die.