Not to excuse the heroine, but from her perspective, I think it'd be pretty easy to jump to the wrong conclusion. If she's actually in the dark about what's going on, she might have only seen Alicia (an acquaintance she doesn't really get along with) rushing forward to kill someone who's trying to flee.
If the heroine had kept her cool or been more observant, she probably would have noticed Jill or Alicia's wounds, and it definitely would have been better to restrain both parties to figure out what's happening. But, well, the heroine's kinda dumb, I guess.