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Due to lack of canonical information in the franchise, there is no definite answer to this question.

Unknown, It could of been that likely that the massacre of the panda's were not to rule china, but get rid of any potential obstacles that would likely get in the way of his possible endeavors to take over china as Shen states that he "had been waiting 30 years for this moment." Implies that he most likely had been waiting to rule china for some time. The panda massacre had been in response to a fortune telling predicting that a "Warrior of black and white would get in his way" if Shen continued down this path, which described a panda.  It was most likely his belief or the notion that his parents, and the world had wronged him and owed him something,forged from his past.In this case, It was all of china.