I only know what I saw in the video, but when I watch I don't necessarily see someone who's native state would be considered "trigger happy." I think it's possible that when you're holding a gun on someone, and the person shouting all the commands is freaking the fuck out and losing his mind, you're probably pretty amped up at that point. "Geez, this seems to be a bigger deal than I thought, he keeps threatening to kill them, guess I'd better be ready to actually pull the trigger here so no one on my team does due to my hesitating." Obviously I've no experience with this, but I can at least see that side of why this happened. It doesn't excuse that it happened, just probably more likely than that PD having a team of blood thirsty murders out running the streets.