Be as nice as you want, as good as you want, but the road to hell is paved with good intentions, and through the dark side there is the inevitable corruption that all Sith and Dark Jedi will find, no matter how good they once were. One should remember that both Jacen Solo and Ulic Qel-Droma were once the most shining examples of Jedi knights, only for both to fall and become Sith Lords - one with a motive to "save" the galaxy, the other seeing a superior and "correct" method of galactic authority.
Both became corrupted, both were killed (though Ulic Qel-Droma was redeemed before he died.)