Imposter?

recently i was indulging in a story and i come across with a character that happened to be an imposter.

anyway, i come across this type of character in this particular story... well, how to say... let me summarize the character's story within the story..

this person was a noble, do a lot of good deeds, but actually he always have this evil thinking in his head. at the end of the story, he finally show his true intention and was quickly identify by other people as an imposter.. and his plan failed.

well, the funny thing is, the guy had been doing good deeds all the way along the storyline.. and at the end of it, the author revealed that the guy had an evil plan.. @@.. and.. then.. everyone in the story was like "owh, he is a bad guy!!!"

they totally forgotten how this so called imposter help them soooooooo much in the story..

is an imposter pretend because they want to achieve their evil plan secretly?

or they are just trying to hide their silly thought?

or they actually know its evil and trying to control themselves?

but anyway.. i feel that those who quickly identify the guy as imposter are way even worst. coz they are just a bunch of people who awez think that they are ethical or morally educated and then think that they will never make mistake.