Suddenly a man came in ----- a gun and said to him,"Your mon

这是用介词with表伴随方式,类似的用法还有：
The man talked to me with a smile on his face.那位男士面带微笑地跟我说话.
Our teacher came in with a book under his arm.我们的老师腋下夹着一本书进来了.
The woman sat on the chair with a baby in her arms.那位女人怀抱婴儿坐在椅子上.
以上的with a smile on his face面带微笑,with a book under his arm腋下夹着一本书,with a baby in her arms怀抱婴儿 都是伴随动作或伴随方式.