It has everything to do with what happened to him. It was not an execution for his crimes. It was a death that stemmed from his own careless, violent criminal actions. Once he drove at the officers, which bordered on attempted murder, and then almost took out a pedestrian on the street, the cops knew he was willing to take them out if need be in his desperate escape. So they escalated their force to meet his prior actions/
As to the fist bump, it is cringeworthy, but I do understand the context. They were 'saluting' each other that none of them were killed and they had each other's backs. It is a life and death situation for them too. And they go through it routinely so the optics are not good at all when they do that and I doubt it will happen that way again. But it was not a fist bump celebrating the young man's death. It was about them not dying that night themselves.
That young man came at them in a deadly way, having no concern for their lives. Once that happens, the cops are looking at him as a deadly threat.
What do "prior actions" of another have to do with being in fear for your life? If they didn't get him that day, do those prior actions put a bounty on his head for an indefinite period of time?
Sounds more like a revenge killing to me than justifiable use of lethal force. But I digress, this thread is in regards to KG, who, although likely a despicable human being, could have been arrested at another time and/or place that would have kept her life, her sons life, and the lives of the men in blue at a much lower risk of injury or death.
And a fist bump is what Kopitar does to D. Brown after a PP goal.

I would expect to see a consoling hug if they were truly celebrating not being killed.