In most RPG:s that allow for these types of choices I tend to play the way I feel I would, or would like to, in real life - which tends to be fairly altruistic and good natured, except when someone really irritates me.
So now that I'm trying to go almost the opposite route I feel I need some kind of moral and philosophical compass to navigate by to not just blindly choose the most evil/nasty option because really what's the fun in choice if you've already made your mind up ahead of time.
I decided that I would use the Battlestar Galactica character Admiral Helena Cain of the Battlestar Pegasus as my template. A military leader with a hard and relentless attitude that shows little compassion, never willing to compromise and is always ready to commit any atrocity that she deems necessary to accomplish the greater good - the survival of humanity and the eradication of any beings that stand in the way of that goal.
In fitting with the themes of Mass Effect - where, colaboration with and understanding of, other races is a key component to besting the Reapers I also though it fitting that she would be a specist, quite in line with the opinions of Cerberus in the second game.
Even with this role-model though I often find it difficult to go against my better judgement. Backing the Terra Firma party with their specist undertones, giving Liara and Tali the cold shoulder and eliminating the Rachni queen where tough decisions that felt so wrong. The old cliche "it's good to be bad" sure rings true in some situations when you want to let off steam, but staying the path of a hardened narrow-minded and compassion-less character is hard. Maybe that's a good thing though, I loved to hate Cain in Battlestar and tough I feel very much a part of my "Cain" Shepard I hope I'm not just experiencing "the other side" of Mass Effect, but that I'm also scrutinizing myself through this mirror image.