Technology

Alternativelly, they've just been competent in the execution of their less savory intelligence operations and thus not been caught doing something too outrageous.

It makes a lot more sense for China to arrange an "overdose" than shoot somebody in the middle of a busy street in broad daylight from a car with diplomatic plates and a Chinese flag.

Same for all other countries, by the way, though in Autocracies politicians have less to worry if the country ever gets caught murdering people in foreign soil than politicians in Democracies do (though, judging by a century of American murders, even those in supposed Democracies almost never have to worry about it)