Most people would be afraid when a gun was aimed at them. I don't think the offensive nature of the joke can be defended. What is interesting though is the crowd laughed, not a huge amount of laughter but some did. It does not sound as if the crowd reacted negitively to Huckabee's statement.
The belief out there that if Obama was president someone would try to assassinate him has been broached by Obama supporters. One fashion designer created shirts asking Who Killed Obama. Doris Lessing stated:
"He would probably not last long, a black man in the position of president. They would kill him,"
Some have stated this fear is a reason to not vote for Obama.
Even Obama's wife has addressed this as a concern.
Which means, while the joke was offensive, those that are expressing outrage that anyone would even think of this (beyond the outrage over how stupid the joke was) seem to be forgetting that despite the stupidity of the joke, the initial expression of an attempt to kill Obama has been talked about again and again.
I'm fairly certain that out there somewhere a black comic has made jokes about Obama or some other black presidential candidate being killed, there was a whole Chris Rock movie where that was mentioned. There have also been quite a few jokes out there that while don't involve death, could be considered offensive:
"In New York, Catholic groups have forced an art gallery to shut down an exhibition of a six-foot image of Jesus in chocolate. Or, as Democrats call it, Barack Obama." --Bill Maher
"Barack Obama was in New Hampshire Sunday. When informed of this, President Bush excitedly asked, 'Did we catch 'em?' --Seth Meyers
Bottom line, over and over again people are selective in what outrages them...