How we cite our quotes: All quotations are from Star Wars: The Empire Strikes Back.
Quote #1
VADER: If he could be turned, he would become a powerful ally.
EMPEROR: Yes, he would be a great asset. Can it be done?
VADER: He will join us or die, my master.
Vader and the Emperor are pretty much hatred embodied. Here, this gruesome twosome is finally realizing that Luke Skywalker poses a threat—and that their only hope for defeating him is to turn him to the dark side.
Quote #2
YODA: Much anger in him, like his father.
OBI-WAN: Was I any different when you taught me?
YODA: He is not ready.
Here, we see that Luke—like his father, and like Obi-Wan—is a bit more passionate than Yoda would like. While passion can be great in many contexts, it can also be twisted towards negativity, as we see with Vader.
Quote #3
YODA: Beware of the dark side. Anger, fear, aggression [...] Easily they flow, quick to join you in a fight.
Although the dark side isn't necessarily stronger than the light side, it is a lot easier to access—all you have to do is think nasty thoughts. This leads to an almost addictive relationship that doesn't end until you're entirely consumed by hatred.