First off, I think it should be said that there's a contradition inherent to the game whether or not you choose good or bad, light or dark, and that is, in simple terms, the world is populated with nameless, faceless goons that you have to blast to pieces by the thousands regardless.
If you get past the sheer number of "people" your character has killed, and go into the RP aspect of conversations, the Empire in particular presents some great RP entertainment value.
I have played a Sith Juggernaut all the way to the end. I tried to RP the character as another take on the idea of Darth Vader, i.e. "there's still good in him." Also as though he was a tragic anti-hero such as in classic literature or plays. My choices were often dark, but also occasionally light. I also tried to take into account the game role that a "tank" class plays, which is to say, he draws enemy fire and enemy ire. He shields and protects allies. Why would a completely selfish, evil monster, protect anyone else? In my mind, it was only logical he might form attachments to the people he was protecting in battle, even become friends with some of them. And, of course, if anyone dared hurt his friends and allies, they would find an unstoppable freight train coming down on them... Whereas a Jedi would have a more philosophical approach, I figured a Sith would be much more of an avenger.
In summary, a flawed human being, a warrior of his people. A true Sith that serves his Emperor and that wants to actually inspire his men, not just make them fear him. He's also the guy in the scary mask with the lightsaber and a penchant for destroying his enemies.
My second Empire character is an alien (Chiss) bounty hunter. For a refreshing change, I made up my mind in advance that this character is quite simply a psychopath, a murderer, an outsider of her people and that's why she's a freelance gun for hire in the first place. She always takes the dark path, especially if it means money, power, and shooting people she finds weak and annoying for fun. Genuinely evil. The BW writing team did a great job with how much you can be a comic villain, full of "moustachio-twirling evil" choices a la classic Disney villains. Frankly it's a blast and quite funny because she's so over the top you can't take her seriously.
My third Empire character is an Imperial Agent (operative) that almost always tends to take the light side choice, not shoot people unless necessary, capture the prisoner, etc. He does his duty, makes the occasional James Bond-esque wisecrack. and flirts with women. I'm only at the beginning of his journey, but I frequently find myself laughing at the humor written into the story. For him, I went with the idea that he grew up on an Imperial colony planet and was inspired by the tales of the founding of the Empire (a la the Journal entries in the holonet). The Republic is his enemy for the same reason that the Russians were James Bond's enemies in the classic movies and novels. They're just the guys on the other side, therefore, "Live and Let Die" as the song goes. All around him he sees Imperial conscripts that need help from a brave soul (i.e, the neverending side quests bailing out Imperial troops everywhere he goes) and they're not really evil either. It's entertaining as I said.