It's always easy to second-guess the cops especially if you've never worked a day as one. I never worked a day as a cop, but I spent 25 years in uniform behind bars, and had to occasionally deal with split-second decisions. No officer wants to have to fire their weapon unless they absolutely have to. It opens up a whole bureaucratic can of worms. The paperwork alone will drive you crazy. The bottom line is that officers have the right to protect themselves and others, as long as their actions are within the guidelines and training of the police department they work for.
I didn't see any rocks in his hand. Are you saying that, after the fact, they get to kill him? Isn't that the same thing as saying, "He tried to stab me but missed. Then he dropped the knife and ran. I still shot him though, because ten minutes ago he tried to stab me"?