Kevin Macdonald's The Last King of Scotland is not precisely a biopic, insofar as its main character is a fictional composite. It's not really a psychodrama, given that the brutal dictator that it takes as subject is never viewed from the "inside." It is a study instead of the seductive appeal of pure evil - the story of a callow white son of privilege falling in with one of the most notorious figures of the 20th Century, but implicitly, also a story about German intellectuals electing an insane anti-Semite, or American Christians falling over backwards to praise a man who views torture as a great democratic tradition.

Real evil is tricky to portray on film. Cartoonish evil is easy - Darth Vader crushing an underling's throat, the Wicked Witch of the West, pick your Disney villain. But actual evil, the kind that sneaks in with a smile, that's rare. Nihilistic sociopaths, raving maniacs, these are nothing compared to the happy friendly face that stabs you in the back, and this is where The Last King of Scotland triumphs utterly, in the performance of Forest Whitaker as Idi Amin Dada. It is a warm and wonderful portrayal that turns into pure horror. The arc of Amin's rise is hopefully familiar: a general overthrows the corrupt government on the strength of his charisma and Everyman appeal; by the end of the decade he massacres some 300,000 political dissenters. Whitaker achieves both of these, sometimes within the same scene, sometimes in the space of a single line reading. So complete is his depiction of Amin the populist that I nearly found myself rooting for him, and even chanting "remember, he ate people - remember, he ate people" under my breath couldn't quite dispel the joy of Whitaker's performance. This isn't merely cosmetic: it is a key component of the end of the film, where our sense of outrage is informed by our sense of betrayal by this kind, wonderful man. Whitaker is a great actor, although his choices have long been suspect; I cannot help but think of Battlefield Earth, where even as he was filled with palpable self-disgust, his was the only performance that could legitimately be called "acting." It's wonderful to see him finally get a role worthy of his talent.

Despite his billing, though, it's a supporting role, as it must be - it would be far too agonizing to watch a film that was all Amin, all the time. The role of protagonist goes instead to James McAvoy's Nicholas Garrigan, a fictional Scottish doctor who travels to Uganda and becomes Amin's personal physician. Garrigan is a discomfiting hero: he is the embodiment of all snotty overeducated white boys who think it's their duty to save the poor brown people from themselves. It's almost impossible to like him right at the start, as he spins a globe around pledging to travel wherever his finger lands - only to find that he has picked the ever-so-unexotic Canada, and so he tries again. We dislike him at this moment, and we dislike him when he becomes intoxicated by Amin's charm, and we dislike his worminess as Amin's crimes pile up; but we identify with him at all times, because he is entirely human. He is a miserable coward, but who among us can truly say that we would be any different in his place? The Last King of Scotland is not a story of a hero daring to be good in defiance of evil, such as Schindler's List; it is about closing your eyes and hoping the bad things go away, and is hence an infinitely more honest depiction of the West's treatment of Africa than most films on the topic.

If the film stumbles, it is that too much of the conflict relies on the actors, and not on the script. Amin does evil, we are told, but we are shown precious little of it, excepting where it affects major characters. 300,000 Ugandans died, but our only real emotional connection is to a handful of people that Garrigan befriends before they are "disappeared." And the end, which basically devolves into a man-to-man fight between Amin and Garrigan cheapens the film's psychology, and muddies its message about white indifference to sub-Saharan Africa. There's nothing sadder and more frustrating than when a great movie gives up in the last ten minutes. There's a distinct sense that the film ends because the writers ran out of ideas to keep it going, and it just stalls.

It seems like once a year or so, we get a film about African suffering: Hotel Rwanda, The Constant Gardener, now this. It's somewhat easy to shrug these off as white Hollywood liberals making themselves feel better, and not entirely inaccurate; but that doesn't make the films themselves any less affecting. And The Last King of Scotland isn't just an Africa message film: it's a sober and adult consideration of corruption and evil. Right now, the story of how a charismatic man ruins his country with greed and terror isn't just of historic importance; in a period when every third film has a specific political axe to grind, this still stands out as a uniquely frightening vision of a decent person doing nothing.