Me, because I felt like it.
I wanted someone to kill that wouldn't have incriminated me if I was found to be Light (I wanted the out of claiming I was good and wanted all criminals dead)
The only reason you lasted that long was because I didn't know if you were one person playing both characters as one role, or what. Once J7 died and confirmed it was seperate, I knew what to do to kill you. Nothing personal :p