As usual, nuance escapes you
A bodyguard may fail to prevent a murder, but that doesn't mean he or she is responsible for it, or aided and abetted, or had prior knowledge, or any other strawmen you come up with.
Having his client murdered means he or she failed to do the job, kinda like Bush failed to do his job to keep Americans safe. Until we get more information, we won't know to what extent he and his administration should be blamed for that failure. It appears that Bush and his administration are more worried about ducking responsibility for their failure than they are about the 9-11 Commission getting a full accounting of the administration's mistakes.
Which only exacerbates the Bushies' credibiity problem.