Emotions and the Dark Side
Firstly, this is an old obsessive interest. When the first Star Wars movie came out (by first I mean in-story timeline, not first to be released - I'm referring to the one where they meet Anakin), I became obsessively interested in that setting, and fantasized about having the ability to control the Force (basically that universe's word for magic).
But even back then, the distinction between Jedi (good guy Force wielders) and Sith (bad guy Force wielders) bothered me. See, according to the lore, Jedi gain power by avoiding strong emotions - seeking out calm and serenity - whereas Sith gain power by feeding their emotions, amplifying them. Even back then, I figured they had it backwards.
See, I was always a deeply moral person, even as a small child. And my moral sense didn't come from cold rationality. Although I've always tried to reason things out to inform my moral decisions, morality is a deeply emotional thing for me. I get upset when I see injustice. I get sad when I see someone suffering. I feel good when I know I've done the right thing - even if it wasn't in my own self-interest. So the idea that emotions are associated with 'the Dark Side' and being unemotional is linked with 'the Light Side' never made sense to me.
Well, turns out my instinct was right. Morality is an emotional thing by nature. I'm not sure if 'evil' as a concept makes sense or is useful, so I'll talk about amoral people instead of evil people (amoral meaning someone whose behavior is not influenced by concerns about right and wrong). In psychology, amoral people are referred to as psychopaths. A number of studies have examined emotional experience in psychopaths and non-psychopaths (see this study for an example) and the most common conclusion is that, in certain specific ways, psychopaths are less emotional than non-psychopaths.
Now, granted, not all emotions are reduced in psychopaths. One emotion that psychopaths feel just as strongly as non-psychopaths is anger - the same emotion that is most commonly referred to in Star Wars as being felt by the Sith. A lot of people, myself included, tend to associate anger with evil. I get scared when people are angry, because an irrational, trauma-affected part of me insists that anger is bad and dangerous and will result in me getting hurt.
But anger is not really an evil emotion. If misdirected, it can cause harm to others, but so can sadness (eg murder-suicide) and fear (eg misdirected self-defense), and we don't tar those emotions with the 'evil' brush. And I've seen my mother and father get angry for good reasons, and direct their anger to accomplishing good things. For example, my father gets angry when people normalize rapists by suggesting that all men have trouble controlling their sexual behavior. (One example was the gang rape of a Cree 12 year old by three adult men who claimed their behavior was excusable because she, while drunk on booze they'd fed her, climbed on the one guy's lap and kissed him. That case got both of my parents angry.) My mother once got so angry at my school for mistreating me that she kicked a hole in the wall while she was on the phone with them.
The most important determinant of how important morality will be to you appears to be how strongly you react, emotionally, to suffering in others. So if generalized tendency to feel emotions has any relationship to morality, it's in the opposite direction from Star Wars lore.
The funny thing is, people know this. A lot of research in morality has focused on dilemmas such as the 'runaway train' model (explained here). These dilemmas pit the good of one or a few people against the good of many. In one version, both the few/one and the many are equally distant (people standing on train tracks while you're at the control booth deciding where the train will go. In that situation, it's easy to decide to redirect the train down the track containing fewer people. But when, instead, the sacrifice involves physically shoving a fat guy in the way of the train, people are more hesitant. Some refuse to say they'd shove the fat guy, even though it means five others dying. Others reluctantly settle on shoving the fat guy as the morally correct, though distasteful, option. However they decide, they feel unpleasant emotions about that situation.
So what if someone else calmly and easily decides to shove the fat guy off? According to this study, deciding too easily to take the utilitarian option, or taking the utilitarian option when the cost is highly salient to the person, results in harsher moral judgement. The reason? It implies that the person isn't deciding for the right reasons - they're not feeling the right emotion. Similarly, children recognize the importance of feeling remorse after doing wrong, since expression of remorse had the strongest impact on their moral judgements of an actor.
I don't know where we got the idea that morality is rational and evil is emotional. Maybe it comes from our desire to convince ourselves that we're better than animals, since animals are as emotional as us but a lot less rational. But the facts show that feeling the right feelings, and feeling them strongly, is essential for morality. You can be great at thinking rationally, but if you don't feel unhappy at another person's misfortune, you won't be a moral person.