Kunzig – Attraction of the Dark Side – Week 3 blog make-up.

The way I understand it, being on the Dark side is not defined by whether or not the character has joined forces with the Empire. It is defined by the possession and use of negative feelings such as anger, hatred, jealousy, or revenge. When a Jedi uses these negative feelings to fuel his powers or abilities, he is in effect using the Dark Force, which is more powerful than the Light Force and can therefore be very tempting.
Given this definition, while Luke has in my opinion never truly considered joining the Empire, he has most definitely slipped into the Dark Side in his efforts to fight Darth Vader. This is because he did not fight Vader with a pure heart, with nothing but the good of the galaxy in mind. He also fought Darth Vader out of hatred, anger and revenge, because he believed him to have killed his father. Real "Jedi knights do not give in to feelings of vengeance" (Pfeiffer 133), but Luke most certainly has, more than once.
One of the instances in which he fought with anger was during his training with Yoda, when he fought an illusion of Darth Vader. The second is when he fought him in person in Cloud City. However, this is also when he learns that Vader is actually his father, and I think this is what saved him – he lost all desire for vengeance since the man he thought had killed his father was his father, still alive. This gave him hope. Obviously he refused Darth Vader's offer to join the Dark Side, because that kind of thing doesn't just happen – a Jedi would have to be tempted into using the Dark Force repeatedly until the line between good and evil no longer matters to him.

Lando also slipped into the Dark Side for a moment, when he betrayed his old friend Han, delivering them to Darth Vader. He did so to save his city, which Vader had threatened, but the means to that end was betrayal, which brings him into the Dark Side. However, he was never fully on the Dark Side, and at some point, probably when he saw Han frozen in a block, he realized what he had done and joined the Rebellion.
Darth Vader is the final example of a character tempted by the power of the Dark Side. Unlike the others, he was deep in it before he was saved by his son, Luke. It was Luke's unwavering belief that there was still good in him, and finally seeing Luke being tortured by the Emperor, that made something click in Vader, and brought him out of the Dark Side in the end of the third movie.