Okay, just to make things a little clearer here since some people appear to have missed a few details this chapter. It’s not just one random maid who’s being held hostage here. We only see three maids tied up and held hostage, one of which gets killed via arrow, but the implication appears to be that ALL of the fmc’s maids are being held hostage. As in, a large group of innocent lives are being held at blade point right now. Plus, one of them was already killed in front of her eyes, meaning she knows the bad guys aren’t bluffing.
Now here’s the speculation part. Notice how old that one maid is. The one who said “Just leave us...!” This implies that the maid has been a part of the household for a very long time. It would even be reasonable to assume based on age and her words that she helped raise fmc. Upon seeing this scene, I got the impression that the old maid isn’t some rando, as she was willing to give up her own life so that the fmc could just fight back. Some random maid is not going to be essentially saying “Forget about us and save yourself”. Again, pure speculation, but it’s what I immediately thought when reading this scene.
Second speculation, the fmc believed that the mc would turn around and leave if he believed he had been betrayed. At least, there’s a chance of that the mc would survive. Remember, the lives of the maids were in immediate danger. MC’s life was not. At the very least, she could play along until she could find the opportunity to save everyone. That’s why she called MC an d idiot while in tears when she saw him fire that signal up into the sky. She was really hoping he’d turn around and run.