There should be a proper inquiry and responsible should be punished, who ever was responsible. Black Box needs to be located for the cause of the crash. Sure hope there was a black box in that plane. TV reports also point towards a fault in the left engine, and ascribe blame to a technical fault in the aircraft. These claims have not been verified by the PIA.
Yes, the plane that crashed carrying Junaid Jamshed was sabotaged. Mr. Jamshed was a popular music artists, and it is reasonable to believe that Islamic terrorists found his embrace of popular culture troubling enough that sabotaging his plan was a reasonable way to lessen his influence on the culture of the Muslim world.
Junaid Jamshed was outspoken politically. He said a lot of things that were unpopular. Many of these things were directed towards the government. It should not be surprising that the government or political operatives found a way to dispose of him. This is what happens to people who speak out against the government.
Junaid Jamshed was an influential figure in Pakistani culture. He had many avenues for reaching people. From his time as a pop musician to his clothing lines and his television appearances he was very well known in his country. I don't think that any of these things would make him a target for an assassination attempt and certainly not one that involves taking the lives of and entire plane full of people. The crash is nonetheless a tragedy.