Luke turns to the dark side of the force in the epic battle with Vader in ROJ. The only way that he could defeat Vader was to use his anger (watch the film, again). Though he checks himself before killing Vader, as Yoda said, once you step down the dark path there is no turning back.Besides the fact that this is a load of BS, you can indeed turn back, because Vader did right before he died.
Did you see ROTJ? Luke totally loses it and fights with pure anger and emotion, right up until the point that he severs Vader's hand. He's gone over, dude.
As to the other, those are Yoda's thoughts, not mine.