Well, the Ming dynasty is not my forte, but the main problem in answering this question is compounded by the fact that the Forbidden City was built during the Ming Dynasty in the early 1400s. That's pretty recent in the grand scheme of things and is beyond my real nitty-gritty knowledge, but hopefully someone more used to the Ming or Qing will come in.

However, if you had asked about assassinations in the various capitals before 1587, one particularly gruesome plot does come to mind from the Tang dynasty that occurred in Changan. A nephew of Emperor Xuanzong, along with three of the nephew's friends, plotted the murder of a man, partly to acquire some of his holdings and partly to settle a grudge (the grudge in question, I'm not familiar with the details of). During the day, they surrounded the man in the palace and bludgeoned him to death, and then proceeded to boil his flesh and eat it. When the throne found about the crime in 739, the nephew was banished, and upon reaching a relay station east of Changan, was ordered to commit suicide.