Many things can cause a BB to curve to one direction or another. Here are a couple for you to consider:
1) Your hop-up is damaged. Check the hop-up mechanism (do a forum search if you don't know what it looks like) and see if you can find any inconsistencies in the construction. Actually, I'm about to post a thread in this forum quite similar to yours, with pictures of what I think are a severly flawed hop-up unit from a KSC G19. I will come back to this thread and throw you a link once I get it done, so you can see an example.
Solution: Replace the hop-up unit.
2) Your inner barrel is wobbling. KSC glocks have a problem with inner barrel wobble, where the O-ring is not quite big enough to create a good friction seal with the outer barrel. Look to the following link for more information.
Solution: Here.
3) Your inner barrel needs cleaning. A clean barrel is essential to a good BB trajectory.
Solution: Clean the damn thing. My inner barrel is short enough to clean with a single sheet of rolled-up paper towel. Just twist it through the barrel until it comes out the other side, and pull it all the way through.
4) Your bb's are too light. Wind and/or the simple light-weight quality can cause a bb to deviate radically, even when fired from an extremely accurate gun. This gets even worse when you use low-quality BB's (foreign no-name brands).
Solution: KSC Glocks work well with (and I speak from experience) KSC Perfect 0.25g BB's. 0.3g BB's are even more accurate, but travel too slow for my personal preference.
Any more questions, don't hesitate to ask.