Why wouldn't Dark Jedi join the Force? Every living thing contributes to the Force. Otherwise there wouldn't even be a dark side, would there?
<{POST_SNAPBACK}>
The Darkside is what the force is not. It's like a hole, which is why it twists people up and makes them unnatural, and make them cease to look human.
Sidious only feared his own death, because he was like a black hole in the force, it was no longer really with him, just his own manipulations with it, so when he died he'd be consumed by this and suffer as a ghost or something like Ajunta Pall. Or something along those lines.