To understand what a person with PTSD goes through "in the moment" we have to think beyond our belief of how we would handle ourselves in a high stress life or death situation. Put self away, go to that place that enables you to kill or be killed.
Forget the theoretical self analyzing the process, but concentrate on the dominating, primeval alpha self that goes beyond rationalizing why or why not, realize that part of you that goes without thinking. This part operates from the law of the wild, the component that keeps you alive when your life becomes threatened to be snatched away. Your will to survive is an entity of its own and will separate from your rationale to preserve itself, self preservation.
We have a filtering mechanism inside of the mind that strains experience looking for the pertinent information needed to navigate stimuli in the environment. The subconscious screens the information through our emotive center which guides us on appropriate actions. When this controller becomes overloaded, the trigger is pulled and the irrational takes over. The flood gate becomes inundated and can no longer hold the storm wind and rain, the dam breaks releasing the rainwater's natural propensity to flow and overwhelm everything in its path.
Once this part of us has been released due to a death threat, it places itself on point and plows the way to safety. That part of us summoned by the heat of anger and the fire of rage and shuts down all thinking and rationalizing to do the deed, the dance of death.