They don't listen to civilians because they are not "authority". Those people with guns are so racist. Just because the alien is tall doesn't mean it's harmful. It's like having a 50 foot tall hippie. Completely harmless. Kinda.
So fun! It's such a funny game, because the zombies spazz out like mad, until you zap them with your gun. Then, they have no more time to spazz. They make these squishy noises that mock me and make me want to kill them more. That's why I like this game. Because it's bloody and slimy.