From a pure vengence standpoint I agree with you, however from an objective standpoint it doesn't make sense. Not only would Cheney get to complete the rest of this term, but most likely succeed in the future re-elections.
All an assination would do, is justify their means of subjecting us to an even greater police state, we'd lean even closer to facism, and they'd pump so much fear into middle america that Big Brother will be that much more empowered.
Imagine what ammendments the Fed's would make to the patriot act if something like that happened...