Of the three generations of Skywalkers thus far, Luke has been the only one to step back from the edge of the darkness. (Okay, fine, Vader returned to the Light, but not until he’d scourged the galaxy for decades and was dying.) That doesn’t mean Luke doesn’t still have a dark side. In Episode IV we saw him display the same characteristics of his father: impatience, resentment, and anger. That carried through into Empire, and was on full display in Return of the Jedi.
When we first see Luke in Episode VI, he is without question tapping into the Dark Side, force-choking Jabba’s Gamorrean Guards, manipulating Bib Fortuna’s mind, and then threatening and trying to kill Jabba. He almost succumbs to it in the Emperor’s chamber, pulled back into the light in a fleeting moment of self-reflection. Luke may have been able to overcome the Dark Side inside him, but it remained dormant and stayed with him, evidenced by the fact that, years later, he momentarily contemplated murdering his own nephew.