I mean if we're being realistic no matter how mentally prepared we think we are for a walking rotting smelling horrifying corpse or hoard of corpses attacking us, if it really happens most people will probably lose their cool. During WWII a bunch of literally dying Russians did one last charge while coughing up blood and pieces of their lungs from a gas attack. The battle hardened Nazis thought they were being attacked by zombies and complete panic broke out. They all fled as fast as they physically could. There are mental and physical training techniques you can do to help ensure you can keep your cool under life and death situations, but at the end of the day you don't know how you will react until the moment comes.