You think the PRC is being antagonistic because it's opposed to a missile system that would provide foreign countries a real-time view of its own airspace? I don't think any sovereign nation of the face of the planet would agree to such an arrangement of having a competing power's military installations so close to its borders, much less an advanced surveillance system like the THAAD.
I also doubt that the THAAD system would make Korean peninsula as safe as you make it sound. The system is designed to shoot down missiles but does not prevent artillery fire, which are NK's "most threatening weapons" which they have 13,000 pieces of lined up on the DMZ.
When the Soviets placed ballistic missiles in Cuba, a country only ~500KM away from the nearest American city, it was the closest we as a species has come to a full-scale nuclear war. So to say that a system that would provide real-time surveillance of a sovereign country's' airspace to another would destabilise the region is not out of the question. It might be a veiled threat on the PRC's behalf but I don't expect any sovereign country to kowtow to another on matters of national security.