They were distracted discussing whether to kill Denji and looking for other ways out. The previous attack(s) also just made it larger, so I can see some hesitation until there are no alternatives, and Aki is bleeding out.
This is a world with devils running amok, so otherworldy horror is not as uncommon as we might think. She is in the business of exterminating supernatural threats, and Himeno said she has the skills, but her paranoia and lack of confidence/fear is abnormal when compared to the rest of the characters in this group. She's unhinged and ready to stab a coworker because a devil told her to.