But this was not a split second decision. She had to see Jean, deliberately unholster her gun, disengage the safety (I assume that, my knowledge of guns is a bit basic the hard bit comes out of the hole) take aim and then fire. She then decided to fire again.
.....
Some pistols, particularly the Glocks often preferred by police, don't have safeties to disengage. But in others it's just a thumb lever; people train to disengage the safety as they draw from the holster. I'm willing to grant that she drew her weapon on instinct; the issue is that she deliberately shot somebody -- twice -- when she had no conceivable grounds to do so, and, in particular, when she could have safely backed out of the unit and demanded an explanation/called for backup/etc.