User blog comment:Zathronas/Speculative author: Who is Summer Rose?/@comment-78.42.186.25-20140809061013/@comment-96.254.157.84-20140809064812

We actually don't know the circumstances of her death. She may have been stabbed in the back by someone she though was a friend (Schnee, IMO) or died of sickness during a mission.

RLR2 only imples that she died while she was away, but doesn't say how.