Max dark side points can take a while, especially if you don't want to anger your companions. It is very easy to accidentally pick up a lot of light side points, so save often and reload. Otherwise, just be evil.
When you do reach full evil, the tracker will be well off of the light/dark scale, and you will receive a bonus above your portrait. I kept thinking the game had bugged and not given me the achievement, but the tracker had further down to go.