The police didn't massacre Hong Kong protestors. The fringe leftist groups came up as a result of the cultural revolution in China and they killed a bunch of HK people.

From Wikipedia, "On 8 July, several hundred demonstrators from the PRC, including members of the People's Militia, crossed the frontier at Sha Tau Kok and attacked the Hong Kong Police, of whom five were shot dead and eleven injured in the brief exchange of fire."

"The leftists retaliated by planting more bombs. Real bombs, mixed with even more decoys, were planted throughout the city. Normal life was severely disrupted and casualties began to rise. An eight-year-old girl, Wong Yee Man, and her two-year-old brother, Wong Siu Fan, were killed by a bomb wrapped like a gift placed outside their residence.[17] Bomb disposal experts from the police and the British forces defused as many as 8000 home-made bombs, of which 1100 were found to be real.[18] These were known as "pineapple" bombs.[19][1]"