Sith are mean dudes. They'll kill you if you so much look at them sideways and they could do it long distance as well, like what happened to that Admiral in Empire Strikes Back. They look cool though with that black hooded cloak they wear no matter which planet they're on. For a secret society hiding from the Jedi Order, they sure like to advertise themselves. But yeah, they're mean bastards.
Also, they live by the "There can only be two" rule. One master and one apprentice at any one time. If someone else wants to join the little group, he/she has to kill either one of them.
See, total bastardry. Not even a courtesy, "What the hell did you do to my apprentice, bitch?"
Scans from 'The Apprentice' collected in Star Wars Tales vol. 5