Most sith channel their power from their feelings, not sure how to explain this but in simple terms some sith are what you call "evil" because they rely on anger , but sith like example Malak have fallen in love and get attached to others.
There actually lot of examples of this because sith can have positive and negative feelings, unlike jedi they dont "deny" their feelings and most sith at some point have get attached or fallen in love, like Anakin Skywalker later Darth Vader did fall in love with Padme and later when he did think he did lose Padme, his only goal was to destroy/find who was responsibly of his loss of love and for this Anakin blames Obi-Wan Kenobi and later he blames himself, this is part of the reason why he becomes Darth Vader . So Anakin Skywalker kind of created Darth Vader because of guilt from thought that he was responsible for death/loss of Padme.
Some sith create alternative personality because of guilt or to seek revenge. Many sith usually become filled with anger and hate because of their past or because of their loss and sometimes they become obsessed with power, usually this is the point where they become obsessed with gaining power they become "evil".
Why has SWTOR portrayed sith differently ? Well unlike movies and other games SWTOR gives more in depth picture of siths then what movies and other games have given in shorter time and because most movies and games show only kind of extreme side of siths and jedi.
Not sure if i explained this well enough.
"Peace is a lie, there is only passion.
Through passion, I gain strength.
Through strength, I gain power.
Through power, I gain victory.
Through victory, my chains are broken.
The Force shall free me."
Also like you see on sith code it doesnt say anything about anger and hate or about obsession to gain power.
Sith code describes that siths ultimate goal should be freedom or/and over come "restrictions" like death.