PSI ratings in StarCluster are limited, leaving a short power range usually capped at about 5 points - which is undeniably powerful, but definitely limited. By turning to the Light or Dark side of the Force, a Jedi can gain more power. Each step towards one side will increase a Jedi's power for those skills which use that side and for neutral skills, and decrease it for those skills which use the opposite side.
Thus a Jedi who has moved by one step into the Dark Side would be 1 point more powerful when using Dark Side skills like Force Lightning and Force Choke, as well as in neutral skills like TK and Shield, while being one point less powerful in Light Side skills like Heal and Stun. The further a Jedi moves into one side or another, the more powerful they become. Say a Jedi has a Force (PSI) potential of 5. By moving three points into the Light Side, the Jedi effectively wields a power of 8 for Dark Side and neutral skills, and a power of 2 for Dark Side skills.
Light Side is not equal to Good, and Dark Side is not equal to Evil. Still, one is what one does. Going further into the Light requires one to cut oneself off from emotion to enhance one's intellect, while going further into the Dark Side requires cutting oneself off from intellect to enhance one's emotion. The sacrifices necessary to move to one side or the other are commensurate with how deeply one goes. The sacrifice necessary to move from step one to step two are much greater than to move from zero (Neutral) to step one.
Evil acts are a shortcut to increased Dark Side power, but not the only way. They are just easier than other emotional paths such as Love. The self-denial required to advance along the Light Side makes one cold and calculating, not caring for others except in the abstract. Neither way makes a Jedi a better person.