Sebastian's Story: The Mortal Instruments (On Hold)

Just to let you readers know, this gives a brief background of before City of Glass but mostly focus' on during City of Glass and the books after. I give all rights to Cassandra Clare, I just wanted to create a story that showed Sebastian's side, as I'm not sure if Cassandra Clare ever did write a story to Sebastian's point of view.
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Everyone knows there are two sides to a story. We've heard Clary's and a little bit of the other characters but what about the bad guys. What about Sebastian, that had no say in his future because it was ruined before he was born? They say Sebastian was a heartless, cold blooded monster but if you have read the series, you would know that there were times when he was actually caring towards his sister, Clarissa, even if they were brief. Did you ever wonder if there was more of a background as to why Sebastian was so terribly, cruel?