Becoming a force ghost is a light side only ability, it's when a user trains themselves to be open with the force and face their inner darkness, then casting it out. They train themselves to become a conduit of the force in their life and retain their identity after death instead of sinking back into the force.
The darksided, menevolent version of this form is called a force aberration, it's more akin to a unstable poltergeist than a guiding spirit. What causes it is the raw emotions and power possessed by a dark sided force user before death, usually it isn't even done on purpose by the sith who have achieved this state, instead of becoming one with theforce, their will becomes more like a footprint in the sand left behind or a stain on a carpet. Their emotional state while an aberration is unstable and bipolar to say the least, they could be teaching one moment and killing you the next.
That's about it in an abbreviated version.
So yes sheev -could- still be a spirit.