It mostly wasn't about trying to overthrow the government with violance. There were still elections in HK for the legislature. But when it appears that mainland central government is passing regulations that reduce any possible legal path for influence politics, then some people react violently to the forcing of tightening control. Such as one thing offered was that governor could be elected but candidates have to be approved by the mainland central government before being elgible for running. Things like trigger a loss of faith in some people about maintaining 1 country 2 system and so the only way to react is violant and destruction. Naturally most of the democracy advocates in HK didn't support the destruction of much of HK. They encouraged protests and demonstrations. Some of the destruction carried was also partly a result of other HK issues such as housing expenses. The main drive for yhe demonstrations and protests was to get democracy sided politicians elected into the legislature. The number of those legislatures did increase in following tbe increase awareness about central mainland government influence that resulted from the 2014 umbrella protests. So very much the legal path towards protecting HKs level of autonomy was being pursued. But the mainland central government did not make clear in a trustful way that 1 country 2 system was going to be maintained. That is not necessarily an argument that 1 country 2 system must be protected for 100 years more and further more. HK has a very long history as part of China dynasty's so it is kind of strange for HK to be a separate system which only left to be that due to left over colony possession. But the mainland's tight government control is something that people not yet harmonized by it tend to want to avoid if at all possible. Quite likely if PRC government control was less then there would be less friction within HK and between HK and the mainland.
What you are describing is what I also thought when i only randomly heard about the hk protest movements pre-2018. I actually was sorta on their side too when i thought it was all rosy. I don't live in hk, but from all the media (including theirs) , social media (of their), random supporters of theirs, i can only conclude it is a full on color revolution/terrorist movement now. Crushing it is the only option.