I don't think proof of this could exist, since you can't look at a corpse and determine whether it belongs to a violent or non-violent protester. Individual eyewitnesses can only establish that some non-violent protesters were killed, but not what the ratio of violent to non-violent protesters among the victims was.
If the Hong Kong protesters want to avoid a repeat of 1989, they'd do well to study the tactics that were tried back then, among which establishing friendly relations with the soldiers appears to have been particularly effective: "Contact with protesters and residents of Beijing sapped the will of the troops to follow their orders, and the food and water offered to them, in lieu of their absent army rations, further diminished their will to act against the protesters." https://en.wikipedia.org/wiki/Insubordination_in_the_PLA_dur...
If the Hong Kong protesters want to avoid a repeat of 1989, they'd do well to study the tactics that were tried back then, among which establishing friendly relations with the soldiers appears to have been particularly effective: "Contact with protesters and residents of Beijing sapped the will of the troops to follow their orders, and the food and water offered to them, in lieu of their absent army rations, further diminished their will to act against the protesters." https://en.wikipedia.org/wiki/Insubordination_in_the_PLA_dur...