There are a couple of points to be made about this "logic". First, if it is taken to its logical conclusion, then we must blame (a) Iran and China for the Iraq war, (b) the U.S. for World War II, (c) Tim Duncan for the Shaq-Kobe feud, and (d) Subway for McDonald's. The fact that no evidence exists for Iran and China instigating the Iraq war, the Americans instigating World War II, Tim Duncan instigating the Shaq-Kobe feud, or Subway convincing McDonald's to use approximately 450 tons of fat in each burger only shows us how brilliant and conniving each of Iran, China, America, Tim Duncan and Subway really are.
The second point to be made is that even if you ignore the ridiculousness of the general "logic" outlined above, it still doesn't make sense in this particular case. As NB mentioned, there are no conceivable set of circumstances under which Musharraf would have benefited from BB's death. He already had what he wanted: the office of the Presidency secured, a pliant judiciary, a muzzled media, the continued support of the West in general and the U.S. in particular, and the participating of all the main parties in the general elections (the PML-N agreed to contest despite Nawaz Sharif not being allowed to run for God's sake). What else could he have possible wanted? And how might have those desires been furthered by murdering BB? To blame Musharraf for this, in my mind, is simply foolish.
Be that as it may, I am not prepared to completely eliminate the possibility of official/establishment involvement. A number of rogue elements within the ISI, in conjunction with some retired generals (ahem, Hameed Gul) and current politicians (*cough Ijaz-ul-Haq cough*) could have conceivably used some nutter to further their objectives by getting rid of BB. But - and this is crucial - the burden of evidence lies on the conspiracy theorists to show it isn't just some Baitullah Mehsud disciple carrying out orders, because that is the most obvious and natural explanation for this. I think we as a nation would do well to familiarize ourselves with the concept of Occam's razor.
All this said, I'm wondering if Pakistanis would have been so quick to blame Musharraf for BB's death if it had happened a year or two ago - before the firing of the Chief Justice (the first time), before the emergency, before the clampdown on the media, before May 12, and before the price of flour went through the roof. Clearly, it's not just about wild imaginations but popularity ratings as well. It is extremely difficult to imagine Musharraf being held culpable for this back when he enjoyed 60% approval ratings. In a perverse way, Musharraf deserves what he's getting here. If he hadn't made so many mistakes, he wouldn't be blamed for one he didn't make.