I think a genuinely mentally ill person is possible, but might not be likely.
Not only are genuinely mentally ill perpetrators rare, but a forum member who once participated in several groups in the sub culture (did not participate in the Star Wars genre) did not describe meeting anybody not capable of distinguishing fantasy and reality.
Of course, the fact that she never met such an individual does not mean that they do not exist. That aside, the forum member did relate that a noticeable minority of participants in her groups got uhmm..... "intense" in regards to pursuing the fantasy hobby interest.
Though not blurring fantasy and reality, these people derived all their self esteem, social status, friends etc. through the cosplay group. Some participants even came to view others as their chosen family.
Likewise, the intense participants could react very badly to the "success" of other participants in the group. The forum member then described a long term friend / participant flying into a jealous rage at her when she concluded that a genre actress (the actress, in fact, had not) gave her a brief personal acknowledgement at a fan convention.
So, while a mentally ill perpetrator might be less likely, there could be plenty of motives from sane cosplay participants.
In the end, how the forum member described her cosplay groups seemed to mirror say, 2% bikers. OK- the cosplayers may not be prone to brawling, but some group dynamics could be similar. For example, the group becoming the "entire world" for some participants. Then, by extension, their standing in that "world" becomes everything.