Anelaid said: She is the catalyst for most of the major events in the story, however.
Having finally watched it up to episode 8, my comments so far: Koishi is a sociopath right from the start (but then again, so is Flandre). Koakuma is into something nastier than simply sociopathy, however. The incident starts rather small but everything goes to hell at the end of episode 5. I had interpreted that at first as Koishi's waves of madness affecting all Gensokyo, but now it can be something different, or THAT and something different.