I believe these people secretly desire someone to really assassinate President Bush and thus put out such suggestive fodder hoping that someone a bit more wacked-out will actually do the job. It is a vile and disgusting display!
I think that a bit part of the basis of BDS is the leftist belief that what they're doing is "Good." Since they're "Good," anyone who disagrees with them is doing "Bad." And of course, to knowingly do something "Bad" is evil. So now the leftists are battling evil, in their minds. Consequently they hate conservatives, who usually look upon liberals as well meaning but wrong headed. It's not the case anymore. More and more leftists are succombing to BDS.
Mark
You have summed it up very well. A lot of politicians excuse bad behavior because, since they are "good" people, anything they want to do to achieve their ends has to be "good." This outlook usually has a finite run as majorities can see behind the self-image of the politician in question and end his tenure in office, or the party's tenure in the majority.
Sometimes, this is a very slow process.