Unsurprisingly, this has been heavily discussed in the Grappling sub-forum. Apparently, the cop was drunk and walked up to Lo's table. He took one of the bottles there and drank from it. He then got in Lo's face and basically challenged him to do something about it.
Lo used minimum force to take the cop down and immobilise him until he seemed to have calmed down. The cop walked away from the table, but then circled back on Lo's blind side and shot him in the head at close range.
It's obvious the cop, who apparently is a Purple in BJJ himself, was looking for an excuse to hurt someone. It's entirely possible that even if Lo had just tried to ignore him, he would have kept escalating until Lo was forced to defend himself. His ego could not deal with being effortlessly taken down by a far superior fighter, so he murdered Lo. A pathetic, cowardly little man who's own fate, I sincerely hope, will be both painful and humiliating.