Just completed what I can only assume is the "light" path of the game... but where does the player make a choice to lead them down the dark path?  I thought maybe it was when you're supposed to rescue Juno, but when I replayed that level, there was no way to just kill her.

I don't know - I never got the impression that there was even a choice to be made.  It felt very linear to me...I'm replaying the game on Sith Master now, I'll keep an eye out.

I think it is the choice at the final battle.

Its hard to say without spoiling it. Spoiler free - its a decision in the final battle.

***SPOILERIFIC***
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If you attack the emperor you get one ending, if you attack Vader you get the other.