Why didn't Marguerite scream?

Marguerite wished to insure her brother's safety..... none-the-less, it wasn't over yet.

To remain--on this spot, without uttering a sound, until I give you leave to speak. Ah! but I think you will obey," he added, with that funny dry chuckle of his as Marguerite's whole figure seemed to stiffen, in defiance of this order, "for let me tell you that if you scream, nay! if you utter one sound, or attempt to move from here, my men--there are thirty of them about--will seize St. Just, de Tournay, and their two friends, and shoot them here--by my orders--before your eyes."

Chauvelin removed he handkerchief from her mouth. She certainly did not scream: at that moment, she had no strength to do anything but barely to hold herself upright, and to force herself to think.