Sir you are ignoring that related history that dates back to the years BEFORE Pakistan was even born. There was no sinister plan to destabilize Afghanistan from Pakistani side.
The unrest in Afghanistan is not something that WE created. People need to understand that there were traditionally two forces at work in Afghanistan for quite a few years. The tribes/people from those northers states, the Uzbek/Tajiks and others. They never adjusted well with the Pashtuns so the differences were ALWAYS there.
Daoud was educated in the west and had somewhat secular views. He did re initiated that Pashtun movement in order to get the Pathans of Afghanistan and Pakistan together and proclaim all that land (that is NWFP). It was more of an attempt to get those people join Afghanistan rather than a purely nationalist movement for Pathans however it did had that adverse effect. Those northern tribes took that as a Pashtun movement and were uncomfortable by the increased Pashtun influence and thus the unrest grew. Remember, there were internal rifts and unrest in the area. Also it is a well-known fact that in the 60s Daoud was responsible for sending troops into our north western areas and Pashtun belt to manipulate those Pathan tribes but they were beaten badly and ran back. We closed the borders and Afghanistan felt the heat because of being a land locked country. He did sort of reunited the Pashtun movement but it the roots can be traced to times before Pakistan was even created and when Daoud restarted it in the 50s/60s it was primarily a move to include our NWFP region and tribes in Afghanistan. Those plans were cut short after the infiltrators were not met with open arms as they would have thought and rather got a beating of a life time and also since Daoud resigned after the new constitution by the King and because the Tajiks, all that time, felt that it will further undermine there importance in national politics so they were hatching plans of there own against the regime.
He did came back later however after he threw there king out, decided to be friend with the communists and started training terrorists for cross border activity in NWFP as well as Baluchistan, again, a very well know fact. However Tajiks and Uzbeks were not very friendly to the regime because of the reasons stated earlier and a LARGE part of Pashtuns were not comfortable either because of those secular and then communist friendly attitude. The communist influence was growing, the Afghans pathans were not happy either, the west was not happy as well. One thing lead to another and we TOGETHER with west started supporting that anti-Daoud faction in order to stop those attacks in our land. The unrest and dissatisfaction WITHIN Afghanistan grew so much that the Ruskies decided enough is enough, their stooge is no longer able to handle the situation and in came the bear! Rest is what we all know!
The reason we supported Mujahedeen’s in Afghanistan was NOT only that they were Muslim Jihadi or that we were afraid Russians will be coming to us next but also because we were part of that anti-Russia/Communist western block! Stating it as if it a Pakistan specif decision and was taken to simply destabilize Afghanistan is unfair and overlooking all the facts.