evil twin

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An identical twin or unrelated person who has a similar appearance to a decent or moral person but who wreaks havoc or does evil.
• noun An imaginary double humorously invoked to explain or excuse uncharacteristic or reprehensible behavior.
• noun A negative or malign counterpart of something considered to be positive or good.
• noun A fraudulent wireless Internet access point used to commit identity theft by recording the data traffic of unwitting users.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic A duplicate or counterpart of something or someone that acts in a contrary, nefarious, or insidious manner.
• noun networking A rogue wireless access point installed near a legitimate one for purposes of eavesdropping or phishing.