I don't think he's intentionally evil.
Just dense.
And arrogant, too. He doesn't get it that just MAYBE, other people know more about the Ring and the Dark Lord than he does. Duh.
*Sigh* A jock caught in a world of geeks. :)
I thought Sean Bean did an EXCELLENT portrayal of Boromir...fleshed him out far better than in the book...in the book he struck me as just a plain old creep, but Bean brought forward some very plausible motivations for why Boromir acted the way he did. When you have grown up as the Golden Son of the highest leader in the land, and theres a good chance that you could save the day by advancing your position, things like an all powerful Ring start to look pretty good...only hubris prevents him from understanding the cost.