Ok, im doing a version of the Infernal Warrior from bioware's site. Now I have reached enough paladin levels, so its time to turn evil!
How do I do this in the OC? Who can I kill etc?

Everybody in sight, I'd say. Perhaps you should have been trying to do a gradual shift to that point, by taking all the not so nice dialog options along the way, if there were any, depends on which OC you are playing. Another option would be to save the char, and then alter the alignment in Chewie's mod, for example, and then run the last save multiplayer, and use the edited version to pick up where you left off.

Do a lot of threatening, mercenary-type acts, too. Where exactly are you in the campaign?