SW: Revenge of the Sith (Did Obi-Wan know?)

[The Mirrorball Man]

^Well, if I remember what I read on Wookieepedia correctly, didn't Luke go a bit Dark Side when Mara Jade died ?

It's the expanded universe, it doesn't count. It's not as if they were trying to be thematically consistent.

 

[Skywalker]

He did go a little bit, but only in the sense that he killed the person he'd believed to have killed Mara out of vengeance, and after that he pretty much shut himself down for a while because of how close he'd let himself get to the dark side.