Many games try to make character development interesting by allowing you to make good and evil decisions throughout the game. Here is a thought exercise: what do you get when a character makes a good choice, then an evil choice, then an evil choice, then three good choices? SW:TOR and many others will tell you this is a good character, completely eradicating the fact that at some point the character was evil twice.
Alignment systems with one axis are pointless. They lack the sensitivity needed to create interesting characters because they only indicate the winning opinion and can’t remember the number, pattern, or frequency of opposing opinions.
If you’re only using one axis to keep a tally of good vs evil, there’s only one interesting decision to be made, once, at the beginning of the game. Will this character be good or bad? Once you’ve made that decision it makes all the calls for you in the future. If you decided to be an evil character then at all decision branch points the evil choices are positive points and the good choices are negative points. If you mistakenly choose good choices your character ends up not evil enough.