Steven Universe - Gizmodo names Pink Diamond villain of the decade

Agreed. Pink may have done some heinous acts in her past, but her fellow Diamonds did much worse. Also, Pink eventually came to realize the error of her ways of her own volition and made a concerted effort to redeem herself, even if she didn't always succeed.

Intergalactic conquest and genocide? Terraforming after said conquest and genocide? "Killing" and then experimenting on gems? Nuking hoards of your own soldiers? Gem based eugenics? etc?
Child's play

You were mean to your pearl and abandoned/forgot about your pet dog when you were a bratty kid?
WORSE. THAN. HITLER!