I think you're wrong about Bush being cast as the one in the wrong in any way, and that the cartoonist thinks there's any "justice", harsh or otherwise, in the politically motivated attempted character "assassination" that I believe is portrayed in the cartoon.
The original photograph captured an actual event. Then it was turned into from an actual event into a propaganda event. For political purposes, the two men in the photo were reduced to archetypes that had nothing to do with reality. Once that happened, no amount of proof would make the shooter appear less guilty or the shootee appear less innocent. They were determined to see this image and disseminate it in a certain way, and in no other. Just as they are now determined to portray Bush as having deceived us all into war. They will lie outright and not think of it as lying, since they have deliberately chosen an alternate reality.
The cartoonist has inverted the situation in that old photo. It's as if he's saying to the left "Hey, remember THIS? It's what you're doing now. And I see you."
It is a Pro-Bush cartoon, but I have to agree it's not clear enough. Poor guy'd better be good at writing, 'cause he's going to have to explain this one tomorrow, and a lot of people STILL won't be satisfied.