He was a spy, and would be shot.
Vincent had so often been in the battlefield, so often under a fire from which it seemed that no one could come alive, that the thought that death was at hand had not for him the terrors that possess those differently circumstanced.

He was going to die for the Confederacy as tens of thousands of brave men had died before, and he rejoiced over the precaution he had taken as to the transmission of his discoveries on the previous day, and felt sure that General Lee would do full justice to his memory, and announce that he had died in doing noble service to the country.
He sighed as he thought of his mother and sisters; but Rose had been married in the spring, and Annie was engaged to an officer in General Beauregard's staff.

Then he thought of Lucy away in Georgia, and for the first time his lips quivered and his cheek paled.
The negro guards, who had been enlisted but a few weeks, were wholly ignorant of their duties, and having once conveyed their prisoner into the room, evidently considered that all further necessity for military strictness was at an end.

They had been ordered to stay in the room with the prisoner, but no instruction had been given as to their conduct there.

They accordingly placed their muskets in one corner of the room, and proceeded to chatter and laugh without further regarding him.
Under other circumstances this carelessness would have inspired Vincent with the thought of escape, but he knew that it was out of the question here.