Lulu. It doesn't appear to be that simple. Yanez, the cop who killed Philando Castile appeared to be scared simply because the presence of a gun was announced as the law demands. It's part of what makes Castile's death so tragic. Castile's partner, Reynolds, is obviously not afraid of Yanez. She instantly recognized the situation was getting out of hand due mostly to Yanez being scared and over-reacting rather than actually listen to what was being said.

It's also why Yanez freaked out as soon as he realized what he had done.