I think anger and fear are for losers and for the weak. These two emotions don't fit with SIth "survival of the strongest" behaviour and their winner mentality. Anger greatly diminishes ones focus and mental performance and consequently increases chance for errors. Winners have no reason to be angry while losers do. As for fear, fear is a powerful weapon so therefore the Sith should induce fear and terror into their enemies, not the other way around. A Sith who is afraid sounds pathetic and weak.
It's something at the base core of psychology. Fear/anxiety/stress/anger etc can be a positive thing unless it's controlling you . Fear can prevent you to be harmed just like it can save your life.. Too much of it can cause panic attack which make you unable to answer to your body reaction. Have your parent ever told you that you don't know danger when you was a kids/teen ? When you learn danger and fear it.. You learn how to protect yourselves from harm .
If you run face to face with a bear. Fear can save you life. Unless you're prepared. Fear often replace it for survival . It's make your entire body reaction change. All the energy of your body is than used only for 1 thing. Survive. Even your eyesight vision change and make you much more focused .
Batman is like a SITh lol. He use his own fear and impose it on others. Being scared/anxious is a normal thing. If you're never scared .. You're a fool or not fully developed. Strong people are scared like everyone else, but they manage to control their own fear or even use them positively. When you're scared of your own body reaction it's when it will control you. You can't control your fear .. but you can control how you perceive it.
Even I will say that the coward are those who don't show their own weakness. Strong people cry and are scared. Overconfidence is a sign of weakness.