Mass Effect's morality system handles "Good" vs. "Evil" in terms of two independent ratings of "Paragon" and "Renegade". It is very possible for a hero who is primarily Paragon to also have earned some Renegade points along the way, though this does not typically detract from his Paragon score.
However, there are (with the exception of some bugs) a finite number of opportunities to increase your score in either of these aspects. Also, a choice to increase one usually means passing up a potential increase of the other.
So, this leaves me wondering out of curiosity: Are there enough morality-based decisions in each game for a player to max out both Paragon and Renegade scales on the same character? If so, do the game mechanics really allow for this?