Why is China worried about defensive missiles in response to Nork threats?

Washington Free Beacon:

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China also has alleged in state media reports that THAAD, along with sea-based Aegis warships equipped with missile defenses, are part of a U.S. effort to set up a networked regional missile defense system.

The Pentagon and Trump administration has remained silent on the Chinese bluster against THAAD.

Pacific Command commander Adm. Harry Harris limited his comments to defending the missile defenses as needed in response to North Korean missile threats.

"Continued provocative actions by North Korea, to include yesterday’s launch of multiple missiles, only confirm the prudence of our alliance decision last year to deploy THAAD to South Korea," Harris said in statement. "We will resolutely honor our alliance commitments to South Korea and stand ready to defend ourselves, the American homeland, and our allies."

Defense Secretary Jim Mattis also has not addressed the Chinese threats and instead limited his remarks to reiterating U.S. support to Japan and South Korea.

THAAD missiles can only be used against offensive missiles and pose no threat to other countries, defense officials said.

On Capitol Hill, Senate Armed Services Committee Chairman John McCain (R., Ariz.) praised the THAAD deployment.

"The arrival of the first elements of the U.S. THAAD missile defense system in South Korea last night is a positive step forward for U.S.-ROK cooperation to confront North Korea’s escalating missile threat," McCain said.

He criticized China's response.

"Unfortunately yet predictably, China responded to the arrival of THAAD with vague threats against the United States and South Korea," McCain said.

"In recent months, China has waged a campaign of diplomatic bullying and economic coercion against South Korea in an attempt to stop the deployment of THAAD. But the reality is that this missile defense system is only necessary because China has aided and abetted North Korea for decades."
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The Chinese probably have same concerns Russia has about missile defense deployments in Europe.  Their primary concern is that it makes it more difficult for China or Russia to intimidate their neighbors.

As a strategic matter, they may also be concerned about the US launching an attack against China and the missile defense system being used to thwart their counter attack.  The Chinese have long seen Korea as a primary avenue of approach by an invading army seeking to conquer China.  That is one reason they responded to the US troops entering North Korea after pushing them out of South Korea.