Clearly they are a were-wolf/cultist/vampire/doppleganger/evil thief and must be slain on sight.
This trope is overused way past the point of burnout.
The original use of this trope dates back to Greyhawk.
"Back in the Greyhawk dungeons, Erac's Cousin and the fighter Ayelerarch came upon a beautiful face that cried golden tears. The face told the adventurers the tale of his imprisonment and the heroic deeds required to release him. The adventurers agreed to recover The Urn of Moon Dust from a group of werebears. Erac’s Cousin and Ayelerach successfully recovered the urn, and to complete the quest, they sprinkled the moon dust on the weeping visage. The face was actually the demon prince of deception, Fraz-Urb'luu, who had been imprisoned by the mad arch-mage Zagyg centuries earlier. The completion of the quest resulted in his release. When he came to his proper form the adventurers attacked the fiend to try to undo their foolishness. The enraged demon fought back fiercely. In desperation Erac's Cousin used a gate spell from a scroll he had and managed to gate in the god Zeus, but to their shock and horror, the god chose to ignore their plea for help. The demon then whisked himself and the adventurers back to his own plane where strange forces there drained the magic from all the items Ayelerach and Erac's Cousin were carrying, including Erac's Cousin's prized Vorpal Blades. Fraz-Urb'luu quickly subdued the stranded adventurers and they suffered unspeakable tortures at his hands before they eventually managed to escape.Erac's Cousin blamed the gods for the suffering he had endured at the demon’s hands, and for the loss of his prized items. He bitterly turned his back on the good powers he had paid homage to in the past, and instead chose for himself a path of evil. The unnamed wizard called upon the archfiend of Hell, Asmodeus, and a pact was made. The one-time powerful force for good, was now one of Hell’s greatest champions. To aid in his endeavors, and as part of their pact, Erac's Cousin was given an imp as a familiar. He does not trust the imp though, and he fears Asmodeus may have deceived him somehow." -Scott, Sep 28, 2007 at 12:15pm, Doomsday Message Board
But this is far, far and away from the only time this trope is used:
"XIII: The room has its own lighting and shows an area filled with cushions of satin and silk. There are no other apparent exits from the room and players can see a winged woman asleep on one of the cushions in a far corner.
ON CLOSER INSPECTION: If the room is paced out, it is forty feet east and west by sixty feet north and south. The female has a belted dagger and no other article of clothing.
NOTES FOR THE REFEREE: The being is an Erinyes devil (HP: 48; #AT: 1; D: 1-4 with venom dagger; AC: 2; SA: Save versus poison dagger) and will sleep unless she is purposely wakened or a loud noise is made in the room. When wakened gently, she will seem kind and ask one of the players to release her from her bondage. All characters will notice a silver chain on her ankle (hidden by a pillow until then). The chain can be easily cut and she will promise anything to get it off. When released, she will try to kill the whole group and will follow them everywhere in this attempt." —The Mansion of Mad Professor Ludlow, Dragon #42
And:
"Priest: The western portion houses the jovial priest who is taking advantage of his stopover at the KEEP to discuss theology with learned folk and to convert others. Everyone speaks well of him, although the two acolytes with him are avoided, as they never speak - the priest says they must follow vows of silence until they attain priestly standing. His well-appointed chambers are comfortably furnished and guests are always welcomed with a cozy fire and plenty of ale or wine. The priest is a very fine companion and an excellent listener. He does not press his religious beliefs upon any unwilling person. He is outspoken in his hatred of evil, and if approached by a party of adventurers seeking the Caves of Chaos, he will certainly accompany them. . . (Note: All are chaotic and evil, being in the KEEP to spy and defeat those seeking to gain experience by challenging the monsters in the Caves of Chaos. Once in the caves the priest will use a cause light wounds (does 2-7 points of damage to the creature touched, a normal “to hit” roll must be made to touch the victim) or a light spell as needed to hinder and harm adventurers. Betrayal will always occur during a crucial encounter with monsters.)"—B2: Keep on the BorderlandsAnd
"The prisoners, thirteen in number, are ragged and beaten looking. Eleven of these are common folk, both male and female . . . while one of the remaining two is a 4th level fighter (hp 20) who will join the party if equipment can be provided for him. The remaining slave is actually a doppelganger . . . who, when accidentally captured by the orcs, decided to pose as a slave while preying on any creature he could find. At times when he is unobserved, he will attempt to slip free of his bonds and hunt for prey."—A1: Slave Pits of the Undercity
And even:
"Roadside Hospitality When the caravan reaches its stopping point for the night, two buxom twin sisters are there ahead of them, setting up camp and tending to their horses. Arietta and Zelina Innevar take a liking to some of the travelers—possibly, but not necessarily a few of the characters—and spend the evening asking about their past, where they're headed, and whether they have family and so on. The sisters are actually two doppelgangers. They can either attack someone that night or in the caravan for a few days while they study the travelers and choose their victims. When the time comes to strike, they wait until after dark, then try to lure their target away from other people by calling for assistance in a familiar voice. Fortunately for the characters and their fellow travelers, if one is defeated, the other flees in a flurry of curses and vengeful threats." - Horde of the Dragon Queen, Episode 4
So, it's not a surprise, really. There are more examples from the original Temple of Elemental Evil, to just about any product published anywhere. You know what would be a surprise? Completely innocent innocents. Or innocent monsters who are openly monsters but don't want to fight the party. Or anything but another doppelganger/evil thief/succubus.
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