South Korea’s deployment of THAAD upsets China, seen as espionage tool

Some is basic international relations (which I'm no expert in, and I'm writing quickly off the top of my head ...):

Fundamentally, there are multiple powers in close proximity (China, Japan, Russia, and currently S Korea) who are militarily competitive (i.e., there's not one clearly dominant power, as in N. America, whom the others can't challenge) and vulnerable to each other (e.g., not separated by an ocean, like the U.S. and other powers). This is a sure recipe for wars, and there's a history of it happening in East Asia. Furthermore, there is a lot of nationalistic hatred, especially between Japan and the other powers. Another example of powers in proximity is Europe through WWII, where wars were fought for hundreds of years.

Domestically, conflicts are resolved through rules (laws) and mechanisms (courts), and there is an authority to enforce it all (government). In the international arena there is no authority; it's fundamentally Darwinian anarchy and nations, fearing for their very survival or seeing opportunity in the lawlessness, end up shooting at each other. The way Europe stopped having wars was to create the European Union, which provided the authority, rules, and mechanisms (and the American presence helped too). Globally, the United Nations and "international law" serve that purpose to a degree, and international wars have been greatly reduced since WWII, but really there is no authority with enforcement power.

But there is, to an extent, one exception to that last sentence, the United States. The U.S. can play that role to a degree. They aren't a legitimate authority, in that nobody elected them and they represent nobody but their own interests (hopefully enlightened), but they have the power to enforce things when they want to. Much of the world, desiring some order and wishing to avoid crippling wars, supports it to a degree. The U.S. is, relatively to predecessors and to potential alternatives (China, the old USSR, etc.), a responsible steward: They want peace, human rights, and prosperity. The U.S. is averse to conquering foreign lands (i.e., to keep them) and long-term wars. When you hear American officials talk about maintaining the 'rules-based international order' (referred to by some others as the 'U.S.-led international order'), this role is what they mean.

That's the role the U.S. plays in East Asia. They have the power to keep a lid on conflict there, and therefore the local powers don't need to build up militarily and to seek every advantage to ensure their survival.

China is now challenging the U.S.-led international order, including many of the rules that they feel don't suit them. The more the East Asian powers see the U.S. hesitate to fulfill its role, such as during the recent Presidential campaign, the more they build up their own militaries (which is happening rapidly) or consider making a deal with China (which is discussed, even in Australia).

So even if US guarantees nuclear umbrella for SKorea and Japan, you never know what will happen. And you don't want to take chances with N Korea there.

China already is behaving aggressively, expanding territory under its control (e.g., in the S. China Sea) and threatening to do so elsewhere (e.g, in the Senkaku islands). They may try to justify their next aggression by blaming someone, but that isn't meaningful.

The same applies to Russia, which already had annexed part of Georgia and clearly had designs on the southern Ukrainian province of Crimea (which did nothing to protect them against missiles). They justify their behavior, like all bad actors throughout history. Look at the Nazi's list of excuses as they snatched territory in the run-up to WWII.

The U.S. and others can't be intimidated into inaction while aggressors like Russia and China keep moving forward, for fear of 'upsetting' their tender feelings.