It's not hard to see how Nicholas or anyone else could be taken in by the gregarious, friendly Amin, whose outer jollity masks a deep-seated paranoia and rage. But it's patently obvious what's about to happen to the Scot from the moment he first meets the newly installed leader and helps patch up his sprained hand.
Nicholas is naïve often to the point of frustration, spending more of his time having flings with a fellow aid worker (Gillian Anderson) and one of Amin's own wives (Washington) than either paying attention to his benefactor's atrocities or even, it seems, actually helping people. You could see his character as a metaphor for the blind eye that the white Western world tends to turn to horrors in Africa, but he's really just an annoying plot device meant to provide the audience an easy entry point into Ugandan society.
Sometimes the ploy works, and many of Nicholas' interactions with Amin are fascinating, thanks mostly to Whitaker's volatile and charming performance, mesmerizing whether he's delivering jokes or threats. Like Amin, he uses his talents to effectively distract from the mess going on in the background.