To get what they want they become the very thing that they despise.
In my ponderings sometime ago I came across this strange association of Anakin Skywalker and the Witch from Snow White. No it is not the simple concept on the face that they are both villains, nor is it the fact that both wear dark hoods (although this is interesting), it is this idea that in striving to achieve what they most desire, both of which are selfish pride riddled desires, they become in essence the very thing that the most despise.
In many ways these two characters are fundamentally different. The Witch has one goal in life which she has already seemingly achieved, that is to be the fairest of them all. Anakin's desire he has never had and that appears to be a longing to shield himself from pain and trial, and consequently he also desires to do the same for those associated with him, but more because he doesn't want to have to deal with it than that he loves them. So one is looking to maintain or recapture her desire and the other is looking for a way to accomplish the impossible that has never been. But they both fall to the same thing in the end. Anakin becomes a Sith Lord that thing he feared so much as a boy and has despised as a Jedi, and the Witch of her volition turns into the most haggard and ugly of individuals.
I believe this is highly representative of souls cankered in pride and self absorption, they can never achieve what they truly want but they will go to great lengths to try, even the destruction of that the have previously worked for (and then had to wear ugly hoods).