It's not easy for a movie these days to really surprise viewers with a great villain twist. Having to get the balance right between laying the foundations that something could be amiss without giving the game away too early (and also not just having someone flip from being a good guy to bad on a dime without it making sense in the narrative), more often than not filmmakers completely skew the execution.
We've covered the worst offenders before, with the likes of Toy Story 3's Lots-o-Huggin' Bear and The Dark Knight Rises' Talia Al Ghul all being prime examples of villainous characters movies poorly disguised. In a way you can't blame them for fudging it a little bit, but there's nothing worse than a film thinking it has a smart twist, only for the whole audience to figure it out immediately.
That said, when a flick does nail it perfectly, revealing that a character you've been rooting for has actually been a villain the whole time, it's nothing but pure cinema magic, and almost guarantees to make a release an instant must-see.