I re-read the beginning and man there is not excusing her, she was aware that they were planning to kill Dion, hell, she was present in the planing!
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She could have warned Yuki before they followed with it, she could have warned Yuki after they when along with it, she could have smuggled some healing and magic potions for him to use, she could have
tried, if it wasn't Dion's luck bro would be
dead.
There isn't any excuse in hell that could absolve her, she is as much of murderer than any of the other two, these writers simply cant understand the weight of one actions, i'm tired of terrible actions being justified by the stupidest of reasons.
Telling a person "Hey, two psychos are planning to kill your friend because they want to coup your party" don't blow a persons cover, at max Yuki and others would think, "Oh, this girl has basic morals, nice", but she didn't, so
she is a murderer.