They're not betraying the west on Hong Kong. They're betraying the Hong Kongers on Hong Kong. The agreement that they entered into wasn't written by the west. It was written by Hong Kongers. What's the excuse for betraying an agreement between China and the people of Hong Kong?
Simply put, even 'the people of Hong Kong' aren't a monolithic entity. The situation remains more nuanced than people often assume.
A recent example. There have been frustrations on the side of China that the Covid situation in Hong Kong is severely worsening and straining it's medical infrastructure.
The frustrations come from the fact that in China, if an official fucks up in terms of managing Covid, they are given the boot, censured, or worse. In Hong Kong, those in charge are seen to get off with a slap on the wrist or less for their mismanagement of the situation.
If China legitimately had as much control as people think, they would have gone in and tossed all those incompetents responsible and had them replaced for their failings. But for numerous reasons and considerations, they don't.
The Chinese believe Hong Kong is a domestic issue they will solve in their own time, using their own means. To the CCCP, democracy isn't the solution, so why would they honour it or any agreement like that mindlessly or based on some intangible obligation? After all if it was, we'd be living in a Utopia here in the West wouldn't we?