What US sees, China also sees. There are few if any secrets that can be kept, especially not anything that has paper trails.It would only mean war if it can be conclusively proven that the act was committed by a foreign power. Besides, it doesn't have to be a predominant political figure, but could be important scientists, military figures, etc. as well. It could be done overseas to make investigation difficult and even a mild sickness could open up a window of opportunity for further infiltration.