The Jedi knows that there is emotion.
There is no emotion, there is peace can be interpreted as the need to understand ones emotions and controlling them rather than letting them control you.
It is not a denial of emotion or a rejection of emotions, that is they do not seek to become emotionless automatons but they seek to understand the emotions they possess and not let their emotions and passions rule them and their actions.
Basically a Jedi is selfless, nonmaterialistic, self sacrificing not greedy, helpful, tolerant, modest, humble and serves rather than rules.
A Jedi should never act through ignorance, or act because his passions and emotions tells him to, he should act with serenity in mind and not let selfishness guide him.
In the Star Wars universe killing is not an evil act, Jedi kill.
A Jedi should however always try other solutions for a problem rather than to rely on his lightsaber as his first choice.
The Dark Siders let their emotions and passions rule them, emotions such as fear, anger and hate often leads to them causing harm in a way which could be avoided,
by for example a Jedi who acts with serenity and harmony.