It's fine to oppose Blizzard, but there's such a thing as picking your battles. Shouting "free Hong Kong" at BlizzCon isn't going to make Blizzard change its mind. It's an event under Blizzard's complete control; it's just going to have security escort you out, carry on as if nothing happened, and edit you out of any clips. At best, you get some news stories about Blizzard kicking out protesters, but Blizzard won't be worried since it had the authority to boot people disrupting its event.
If you want to do something meaningful, boycott Blizzard games and encourage friends to do the same. But protesting at BlizzCon strikes me as incredibly short-sighted and more disruptive to attendees than to the company.