I was playing as Dark Side the whole game. I got to the part where you fight Bastila at the Temple Summit, and when I chose to kill her after doing all the Dark Side responses (kill her as a Dark Sider because she just tried to kill me, which is very Dark Side), the game gave me so many Light Side points it took my full Dark Side character, made him Neutral, and forced me into the Light Side ending.

Not gonna lie, soured the game a bit for me.

This was the thing that broke the suspension of belief?

How about at Dantooine when you are speaking to the Jedi Masters and they tell you that they are glad that the secret location of your training temple has not been found out and the next sentence they tell you that many of their students have joined Malak?

Surprised you didn't reload at that point.

Remember, although killing someone is a victory, turning someone to the dark side is an even greater victory. Technically Bastila has already been "turned" at that point, but nevertheless, she's fun to have around as a dark-sider. Plus you'll get to some of your companions (that don't follow you to the dark side) soon after that.

You've definitely missed out on some of the best dark-side moments in the game, e.g. the stuff with the thief girl and the wookie...