I don't think the Sith fear the lightside at all.
The Sith are much more confident, and IMO more powerful, they mostly can do whatever they want, not to be commanded by some green monkey 馃槢.(No offense)
Look, take humans in our world for an example. Would they fear of becoming a bad-all in all jerk, greedy person? Yeah. Would they fear to be a good person, which fights for the right cause? No.
Same thing with the Sith.
It's not quite like that. See the one falls to the darkside, meaning you wouldn't have any control over it, but one has to conciously make the decision to be lightside. The Sith don't fear the lightside because they have control over what side they are on. Like Fishy said, it's easier to fall to the darkside than the other way around.
think of it this way( in earth). how much good can one person do. clean garbage? stop fights? give money to beggers? but one bad person can do a crap load of damage to the world. they could go to every forest and start fires. throw bombs on oil boats. in order to do much good in the world you need lots of money. not every person is as rich as bill gates. if someone bad was as rich as bill gates they could i bealive make this world a really bad place.