The people at the christening party learn that the second stranger is the hangman, and they assume that the third stranger is the criminal whom the hangman is scheduled to execute. To these folks, the first stranger is a nobody. But they see this third stranger's pale face, trembling knees, and shaking hands, and when they witness him bolting from the house, these party attendees make up their minds that he was definitely the criminal and they should organize a party to chase after him. After all, if someone runs away, that means the person is guilty. Right? Maybe not.
As readers who, unlike the party attendants, are probably paying very good attention to the details and not drinking mead or dancing until we're dizzy, we have a pretty good idea of who the first stranger really is! Although we'd have to guess about who that third stranger was and why he arrived, we've got a very good case here for pinning down the first stranger as the real criminal. Let's take a look at our clues.
Readers do have an unfair advantage over the partying folks; we got to see the first stranger from outside, as he slowly approached the house.
We see that this stranger walks purposefully, that he strides “through the rain without a pause” with a regular tread that has “caution in it.” If that doesn’t sound like a wily criminal to you, I don’t know what will! By explaining how his manner of walking is both purposeful and careful, we know that something sneaky is going on with this guy.
Then we see him drinking rainwater thirstily from a vessel placed outside, and then he pauses to think before knocking on the door. This sounds like the behavior of someone who’s been travelling for a while, escaping even, and now he wants to be sure he’s entering a safe place.
When the family lets the stranger inside, he doesn’t immediately take off his hat, as would be polite, so we wonder what he’s hiding. And check out this important sentence:
“His manner, which, before entering, had been so dubious, was now altogether that of a careless and candid man.”
Somehow, this stranger has skillfully and swiftly put on an act. He went from having “dubious” manners while sneaking around outside alone, and now he has “candid” manners while at the party. The change happened in a split second. So we know this stranger has skills in deception.
As he makes himself at home—again, it’s probably a show of false confidence, since we know he was just outside, carefully skulking around and looking for safety—he answers the hostess’s questions vaguely and then gets her to quit asking them through flattery (by hinting that she’s young):
"'But you would hardly have heard of me,’ he said quickly. ‘My time would be long before yours, ma'am, you see.’
This testimony to the youthfulness of his hostess had the effect of stopping her cross-examination.”
Then he makes up excuses as to why he’s a smoke who has no smoking implements with him.
And then, look at the careful, measured nonchalance with which he interacts with the second stranger, having learned that this second fellow is the hangman scheduled to execute him:
"The stranger in cinder-gray took no notice of this whispered string of observations, but again wetted his lips. Seeing that his friend in the chimney-corner was the only one who reciprocated his joviality in any way, he held out his cup toward that appreciative comrade, who also held out his own. They clinked together, the eyes of the rest of the room hanging upon the singer's actions."
All these clues point toward someone who is escaping, and who now is hiding among these strangers. He’s definitely the escaped prisoner! We find out later that the third stranger was the prisoner's brother, which explains how quickly he departed upon seeing his doomed brother.