The wise words of a certain man: "People die if they are killed."
But Allen found out that also worked on aliens that disguised as humans.
So, Allen's wise words are: "If you shoot humans at the head, they die."
Allen found out that shooting aliens who had disguised as humans would die from having their heads being shot and exploded into pieces.
It seemed that the aliens' human transformation was too perfect that it perfectly replicated the limits of a human being.
So if a human dies from being killed, does an alien disguised as a human. So if a human dies from being shot in the head, does an alien disguised as a human.
*Bang!*
The sound of a gunshot reverberated in the open streets as an average human's head exploded into pieces as its headless body fell to the ground, alerting all the humans within the crowd and causing mass panic.