THAAD is one of the most capable ABM systems out there, and fielding it near China will definitely induce panic within Chinese circles and draw their criticism. However, what to do about North Korea?
IMO, US and China should work together to disarm North Korea and unite the Korean countries. In this manner, arms race in this region will diminish and US will have much less incentive to stay there. In the end, one less front to worry about for both stakeholders.