Idi Amin was a butcher and a monster, taking the lives of over 300,000 of his fellow Ugandans in his reign of terror during the 1970s, along with committing countless other atrocities. Yet, "The Last King of Scotland" is not a historical document. It observes the insanity of Amin through the eyes of a corrupted soul, leaving just enough space between fact and fiction to flesh out a fascinating motion picture.

"Scotland" reaches out to find that glint in Amin's eye that made him such a beloved leader when his regime took power in 1971. The man was a childlike charmer, using his hefty political might and wild bits of personality to win over his country and the worldwide press, weirdly alternating between intimidation and adoration at the drop of a hat. "Scotland" finds prime dramatic real estate in just observing Amin, and to a much larger extent, Whitaker's thundering, wild-eyed performance.

The imposing actor captures the hair-trigger patience of the leader, but also pays strict attention to the seductive side of Amin's generosity and friendship. It's a sweaty, bracing performance of a difficult man, and because the feature doesn't keep Amin in the foreground (this is Garrigan's story after all), it can get away with a blurred portrayal that is light on the finer details of Amin's rule, but heavy on his chilling menace and pill-popping fury.

To successfully get inside the mind of Uganda, former documentary director Kevin Macdonald ("One Day in September," "Touching the Void") employs zooming cameras and grainy, splendidly colored cinematography to open up the screen and let the viewer see Amin and his blunt, controlling ways. "Scotland" can be crudely drawn, but uses its low-tech instruments to erect authenticity in the surroundings, not unlike a documentary. When it wants to be, "Scotland" feels immediate and suffocating, painting a stunning picture of Garrigan's naiveté, and eventually his consuming desperation to escape the clutches of Amin.

In a bind to start mounting the evidence of Amin's viciousness late in the film, "Scotland" takes a hard left turn toward gruesome violence in the third act. It's truthful to Garrigan's slowly stewing horror, but the film can't quite manage the strain going from a psychological study to a full-blown horror movie.

Events become unspeakably ghoulish and borderline abstract late in the game, inciting a lack of interest in the story, at the same time, instilling unexpectedly newfound empathy in Garrigan's fight to flee Uganda. Macdonald encourages composer Alex Heffes to charge hard with his score, pushing the anxiety to something more akin to an inflated Hitchcockian romp rather than an unsettling historical drama, leading the film to flame out. Insanity ruled during Amin's reign, and it clouds Macdonald's detailed concentration, forcing "The Last King of Scotland" to detach from its effectiveness, but not its overall authority.