Like Angel and Storm, Psylocke is traditionally a member of the X-Men, not a villain. Although (like Angel), there's comic book precedent for Betsy Braddock serving as a horseman of Apocalypse. In typical comic book fashion, it's a complicated and convoluted story taking place in an alternate timeline (Age of Apocalypse) but the precedent is there.
More interesting is the fact that we now have Angel, Psylocke, Deadpool and Wolverine all existing in the X-Men universe. The only individual missing from the black-ops version of X-Force put together by Wolvie is Fantomex. It will be interesting to see whether this particular group will be explored by 20th Century Fox.
Since the dawn of civilization, he was worshipped as a god. Apocalypse, the first and most powerful mutant from Marvel's X-Men universe, amassed the powers of many other mutants, becoming immortal and invincible. Upon awakening after thousands of years, he is disillusioned with the world as he finds it and recruits a team of powerful mutants, including a disheartened Magneto (Michael Fassbender), to cleanse mankind and create a new world order, over which he will reign. As the fate of the Earth hangs in the balance, Raven (Jennifer Lawrence) with the help of Professor X (James McAvoy) must lead a team of young X-Men to stop their greatest nemesis and save mankind from complete destruction.