User blog comment:Zathronas/Speculative author: Who is Summer Rose?/@comment-71.56.216.102-20140815031147

Im just shamalan-ing here, but a twist that has been used in anime before and would still be if the death of a mentor caused the student to become evil. So it might be that Summer sacrificed herself for Cinder because Cinder was involved in some agenda causing agents for that agenda to chase her down. The result is a dead Summer, causing Cinder to become evil or rather to enact vengeance.