i think 80% off my characters could fit this... I cant help it... but my most notable in the past would be ones like;
James, this guy was some high class brat in england who took advantage of chaos that spread across the world and pretty much indirectly destroyed his entire family. So he kinda dropped off the face of the earth during the chaos that happened (for rp continuity reasons) when the chaos died down he already had connections with the underworld and pretended to be normal and likable until the start of the story (this guy had a real black tongue) after the story kicked in and the action started he was about to abandon his 'team' to side with what was said to be the bad guy, but he was forcibly stopped which lead to him distrusting his side... he is slated to be a villain in a future arc... thou thing is he acts like a real nice guy and doesn't react to provoking.. nor does often get his own hands dirty

Ven, this guy was straight up screwed in the head, murdering family, jeopardizing other RPrs and that kinda shizzle, even got to kill some RPrs... not randomly with legit battle of course.... and they didn't die peacefully oh no, i enjoyed writing down in detail how they where killed.

[some elf dude that had the name that meant mind rape in elven] shar'dnerr or smthn... this guy was once very sweet and innocent, until the fir- i mean until an experiment to implant a more powerful seal (powers in the rp) went wrong and voices of the dead invaded his mind and corrupted him beyond anything, went off to kill his parents for fun, killed the researchers and then had a kinda form of multi personality disorder, as his name suggests he was feared for his powers as he would invade the enemies mind and corrupt it and sometimes able to control it (really all he had to do is project is inner mind and people normally went batshitcray) though due to him being a valuable asset in the war he had a vice-commander position (basicly to have a leash on him) 
he was a sadist, suspected necrophiliac, rapist, masochist (wait what how does that even..) and other less troublesome things 

and one of my personal favs
Savan, now this guy, his entire history was written to make everyone believe he was basically a holy knight who would fight for humanity till the very end.... oh how wrong they where, little do they know this guy is basically on the same level as the main villains ... actually he is a main villain CO-GM POWER INVOKING FTW! though I am surprised no one has ever picked up on the hints i dropped... or a report to a mission i added in the bio where he was the only survivor and there seemed to be issues with him

most my other crazy characters where just misunderstood ;( [s]so what if they murder large groups of people for fun[/s]