For a single moment, the Dark Side had reared up and taken hold of Luke. The fight had been intense, driving both men further along their path, dancing as puppets on Palpatine's strings.
Luke wanted to save his father, and his father was pulling him down the same path that had consumed Anakin Skywalker.
It took Palpatine nearly killing Luke for Anakin to see the full truth of it, to defy this twist in fate.
For the last time in his troubled life, Anakin threw off the shackles of his slavery, and saved his son, and truly balanced the Force.