I would think if you were into Cosplay and intended to kill her, the last thing you would wear is Cosplay gear....
Objectively, what you say is very true.
Humans, however, do not always think objectively regarding any number of things, including criminal actions. Rather, they can think and act subjectively.
As a result, they can do things that would appear to be counter productive and even contradictory to somebody looking at the event from an objective point of view.
In regards to the possibility of a cos player in partial costume, such an individual could be so accustomed to wearing partial costumes of various sorts and being around others who affirm it that they subjectively forget that such a practice is is non existent outside their small sub culture.
Or, though they are not confusing reality and fantasy
per se, they have subjectively become the quasi ultra ego character represented by the costume. As a result, the costume is no longer viewed as really a costume. Rather, the gear is well is every day attire and just.... what
they wear as an individual.
Or, the partial costume represents unity of a small group, sort of like street gang attire. Objectively, the last thing a gang would do is commit a crime decked out in gang gear. Subjectively, however, the gear represents who they are.
Though two or three 'dissed cos players acting as a quasi gang is probably not very likely, one does not need to be an urban youth to form a gang like mentality and have "badge clothing". An example would be middle aged men with good incomes riding around with a "2% " biker club.