The Dark side corrupts you bit by bit every time you tap into it. It warps your mind and perception. "The road to Hell is paved with good intentions" is very true for a 'good' Sith. Eventually they will become monsters, no matter how benevolent they might be initially.
The Irony is strong in the Sith. They are led to believe that the 'Force' will set them free, when in fact they gradually become slaves to the power they worship.