Dark Jedi don't need to use the darkside to be dark.
Dark Jedi are just former Jedi who have turned away from the Jedi Order and live their lifes how they see fit without some order telling them what they can and can't do. If they want to love than they love. (That can turn to anger and greed and all that razzle dazzle and usally end up with the dark side of the force being used) But any jedi who doesn't follow the Jedi code is a dark Jedi. They don't need to use the darkside at all. It happens in some cases but...you know....
Jedi - Good
Dark Jedi - Neutral, They can choose what to do.
Sith - Evil