That's exactly what they're trying to do. In this case, they could have made the law such that any extradition would have to go through the HK legal system, and

That's exactly what they're trying to do. In this case, they could have made the law such that any extradition would have to go through the HK legal system, and mainland China would have to present enough evidence for a HK judge to come to the conclusion that the person should be extradited by HK law. But they didn't as far as I have understood. So by forcing this law, and forcefully silence the protesters it will be clear that they're not going for the 1 country, 2 systems; but rather 1 country, 2 systems except for ... until it's just 1 country.