The Sith assassins can't use the Force. They've had the same training Atton did to shield their minds from Jedi, and the technique also blinds them to the Force. They throw up walls of emotion that mess with a Jedi's senses because Jedi have to block that out to use the Force. Consequentially this makes it difficult for the assassins to use the Force themselves. They do use the Force in the sense that they use it to become invisible, sense their prey, weaken them, etc but they are not typical Force users. Essentially they specialize in negating the Force, and it would also negate their own Force.