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.
You sound like another well meaning white colonizer (even if you are Japanese)
First, your invocation of Liu Xiaobo basically discounts everything you said. That guy was a nut case. This guy advocated for the very violent overthrow of CCP by America and their military allies and used wars like Vietnam and Iraq to justify it. The United States certainly committed far worse crimes against the Vietnamese people than the communists ever did, yet this guy thinks it’s defensible. Not a great example...
Second, as people pointed out, most of these people are not “pro democracy”, this is a label given to them by western/western-aligned media. Their xenophobic undertones of attacking mandarin speakers, hokkienese, pro-PRC people shows their true colours.
Anyone with just a shred of historical knowledge knows, the Nazi party was democratically elected.