User blog comment:Aegis Runestone/Does anyone else dislike Lucy being... *spoilers*/@comment-200.220.180.66-20120417143943

It's understandable.

The amount of stress Lucy's been through, not being able to contact her friends, family and William's coldness made her defect, but hey.

She accepted the mission to stay undercover, to feed information. She should've known William would treat her as a Templar to avoid suspicion, but at the same time try to help her the best way he could. So, when he found out he clearly stopped caring. There's too much at stake for the Brotherhood to accept weakness from one of the members.

She was weak, she gave in, she made herself to believe in a false love by the Templars, betray the ones who took care of her since her birth, while in the end Lucy was just collateral. She's a tool to be used by the Templars, and the way Vidic treated her reflected that too well.

TL;DR: she had it coming.