Since it's the same source that establishes that Siona killed the Chantry sister - the thing you've argued makes the Elves the only bad guys where the Red Crossing is concerned - does that mean you now say the Chantry sister's death couldn't possibly have happened?
Ok, I had left this disccusion actually, but i'll bite.
Please note that the writer of this scroll serves as both realiable and an unrealiable narrator.
In regards to Siona's sister, he was not present as the events that led to her death, and only knows she is dead and her body was bought back by by one of their fellow knights (who was about to defect).
As such any claim or allegation he makes towards the nature of said death is merely speculation borne of biased suspision, not fact, and as such cannot be considered binding as truth or reliable in any form.
In regards to the attack on Red Crossing he was in fact present to witness the event in full, thus making this an account of events entirely realiable as he witnessed and pariticipated in the murders and red crossing.
As such, despite being on the same scroll, both stories are in fact not equal.
So yeah, proof please?