Since Shallan has so much back story untold, I have a feeling that in book 2, there's a good possibility that Shallan will become the primary protaganist, like Kaladin in this book. That way the flashbacks can tell her father's mysterious story and the way she got her shardblade and how the soulcaster got cracked and where her oldest brother, Helaran, (who probably ISN'T dead) disappeared to.
I don't think there's much to ponder there. Who would expect their beloved daughter of killing them? Getting him dead within the 10 hearbeats doesn't seem like a big stretch.
I decided to find the little hints of flashbacks in chapter 7 and 8 to see what we can figure out about the event.
It still felt odd to her that she been the one to take charge after...After the incident...After..
Memories attacked her. Nan Balat bruised, his coat torn. A long, silvery sword in her hand, sharp enough to cut stones as if they were water.
I guess if the memories are of after the incident, then she probably got the Shardblade from her father.
Speaking of which, is that a typo in the book? I think that it should be "It still felt odd to her that
she HAD been the one..."
And then talking about the soulcaster...
Nobody knew where he'd gotten the fabrial, which she now carried in her safe pouch. It was unusable, damaged on the same disastrous evening that her father had died.
This quote, on the other hand, seems to imply to me that the Shardblade played a part in the damaging of the Soulcaster.
So... basically, I have no real theories yet, however, Brandon probably has hidden a big secret in the events that really transpired that day.