Why would they be looking for an excuse to execute him?
To be fair, the sound immediately before the shooting makes it looks like they were very suddenly startled by something, you can even hear them shout "drop the gun" or something like that. I can't really blame them for being trigger happy when someone reaches for their gun, if that's what happened above. In fact, I think reaching for a gun in any circumstances in front of a cop, in the USA, should be akin to suicide.
Statistics alone show how dangerous the job is, and the dozens, dozens and dozens of youtube videos like the ones
@TheReligion posted, give you a very grim picture of how quickly and suddenly these things start and how quickly these things are over. Before I started watching shit like this I really didn't realize how quickly things change from life and calm to panic and death, we're all too used to watching dramatic slow representations of events in movies.
It's a messed up country in so many way. Cops die like in the videos above because guns are everywhere, and because many of those soon-to-be murderers are probably on the verge of some ridiculously long prison sentence for drug possession or something like that.
I don't sympathize with trigger-happy cops, but then I don't sympathize with the mentality of the country as a whole, and I don't think one can analyze their actions without that context.