How we cite our quotes: All quotations are from Star Wars: The Empire Strikes Back.
Quote #7
[Luke decapitates a manifestation of Darth Vader in a mystical cave on Dagobah. The head explodes to reveal Luke's own face beneath the helmet.]
This striking image (which scared the Ewoks out of us when we were kids) represents Luke's hatred and anger towards Vader, which could lead him down the same dark path as the Sith Master if left unchecked. Of course, it also foreshadows the later revelation that Vader is Luke's dad.
Quote #8
OBI-WAN: This is a dangerous time for you, when you will be tempted by the dark side of the Force.
YODA: Yes, yes. To Obi-Wan you listen. The cave. Remember your failure at the cave.
Both Obi-Wan and Yoda can see that Luke is in trouble, but they're unable to convince him to remain on Dagobah and complete his training. Luke is still too emotional—too angry—and they worry that this will make him rife for conversion to the dark side. Let's just hope that Luke was taking notes during Decapitation 101.
Quote #9
VADER: Obi-Wan has taught you well. You have controlled your fear, now release your anger. Only your hatred can destroy me.
On some level, it seems like Vader would be totally cool with Luke killing him. After all, this would represent the young Skywalker fully giving in to his hatred towards Vader and, by proxy, turning to the dark side of the Force. Like father, like son.