Not how i see the video.

The cop draws his gun on the approaching masked man #1 in which i see as an attempt to get him to back off.

Instead of backing off masked man #1 keeps approaching and squares up to the police officer, after which the police officer tried to restrain him, this is when masked man #2 approaches and appears to be attempting to grab the police officer's gun, he might have just tried to get it to not point at him instead but it could easily be interpreted as the former. I think in most countries attempting to grab a cops gun in a hostile situation results in getting shot. The treatment of the man after getting shot seemed appalling though.

Please don't think for a moment that i do not support HK or not have sympathy for the poor guy getting shot but i think the cop was in a scary situation and felt threatened and the behavior of the two protesters surely did not help the situation.