Yes, he probably would have been decapitated even had the US not gone to war with Iraq.
You may not keep up with the news, but Muslim extremists in other parts of the world have decapitated American hostages several times in the last few years, prior to this war.
And Berg seemed to be at the least likely to put himself into a trouble spot somewhere. His death is his own fault, and the only blame that can be laid is at the people who actually committed the act of killing him.