Also the Sith WERE able to explore all the Force had to offer ever since the Jedi Schism. They were exiled to some wayward planet where they did s they wished with their powers of the Force i mean since the schism there were several grps/cults of Sith. The Sith Brotherhood(Lost Tribe of the Sith Series and the Bane Books cover this Sith grp) Exar Kuns Sith, Freedon Naads Sith, Naga Sadows Sith. they may be all in the same i think but this is just a idea of their origins since the Schism.
So there was no real lack of freedom for them to explore all sides of the force just exilement for wanting to use emotions to gain power which the Jedi were against.
the ability to use ANY Force technique i would say is inherent in using the Force. but to use the Force to cause harm to others freely for pleasure and enjoymentand not in defense is what the Jedi would consider Darkside use.
Well for me the Sith always had a strong/weak dominant mindset.The Sith believed the Jedi weak for not allowing the use of emotions when dealing with the Force and believed that those without Force powers were just plain weak and not worth living.