Ok you have a few choices for "Evil Characters" and ill lay some out for you.
-Origin-
Human Noble, more Neutral than anything else.
Dwarf Noble, can be made very Evil, make sure to kill Trian yourself.
City Elf, can be made a very evil murderer.
Dahlish, Neutral/good
Elf Magi, can be racist as a elf and a mage. kinda a jerk
Dwarf Commoner, Murderer
Human Magi, Neutral
I will suggest certain origins for certain builds.
-Warrior-
Simplest is the
Evil Overlord build. Which is just a Two-Hand style in heavy armor (Think Sauron from LOTR). Important specs. I suggest Templar/Reaver or Reaver/Templar. We will ignore Berserker and Champion as they are Stamina draining specs and two-hand is all about spamming stam. (Im working on a Berserker Mod that will make it reduce your health instead and boost stam regen). This is a deadly and evil themed build but really comes into it own in Awakening where you can go Reaver/Templar/Spirit Warrior and not only do huge damage but also get 100% Dodge Rate and 100% Spell Resistance. Truely a Nightmare.
^ I suggest Dwarf Noble, or Dwarf Commoner for the better racials, but City Elf can be more roleplay and story satisfying (Since you can be racist when your a Elf)
-Magi- Always Elf
The standard Arcane Warrior/Blood Mage works fine. Can also be done with a Bloodmage/Spirit Healer or a Dedicated Shapeshifter (Bear/Spider Form always). The Elf origin lets you be mocking and racist as usual but you can also lord your magi power over others. Also alot more options open up as a mage when dealing with demons and you should side with them

-Rogue-
Any build really. Str based Duelest/Ranger/Legion Scout probably be the simplest "Thug" type but. You can also build a Cunning based Bard/Assassin/Shadow and do great. Im currnetly experimenting with a roleplay/test build Ranger/Assassin/Shadow build myself.
^ Dwarf Commoner, City Elf are the best synergies I think for a Dark background rogue.
- Evil Party -
Morrigan, Ohgren, Lohgaine. Can also sub in Dog or Shale though they are more nuetral they will still agree with most your evil actions.
-Plotwise-
You can either be evil incarnate or Ruthless. There are some differances and some options require mods. Like a Blood Mage can actualy kill all the circle and templars and not side with anyone with a certain mod that restores that option. Ruthless happens to coincide with the True Warden path so i prefer that.
But mainly
make deals with demons and blood mages.
side with werewolves
side with Bhelen/Branka= Ruthless, Harrowmont/Branka=Evil
side with Mages=Ruthless, Side with Templars=Evil, Side with neither= really evil
side with cult of andraste (too many options to get into details) =Ruthless/Evil/Really evil
Anora on throne and spare lohgaine, execute alistair= evil
Anora+Hardend Alistair on Throne, Lohgaine spared=Ruthless (and probably the best outcome)
Do the Dark Ritual (It just opens up so many plot options in the future I dont see why not)