"I won't kill you... but I don't have to save you." "From Vader? Hypocrite." "We all are hypocrites, but some of us try to be better about it, unlike you... or your master."
In a word, a Jedi has something a Sith doesn’t, a Soul. Sith do nothing but feed into their primal natures and base emotions; making them slaves to their own whims. Jedi are stronger because they have the willpower to hold back. A broken Jedi may be scarier, but they’re hardly stronger. They just kill things faster, not easier.