“so I really don’t get it...”
With something like a pandemic, nuclear attack, etc. the people that are infected/contaminated would have no chance of recovery, they are the walking dead, and if you come in contact with them you will also become infected/contaminated, thus you too will become the walking dead.
To stop the walking dead from causing more death by spreading the disease/radiation to others you kill them.
I recall hearing it discussed when I was a kid during the Cuban missile crisis.
My mom and aunt were talking about things like water, food and ammo, bringing livestock in closer at night, what were the chances that desperate people seeking shelter and food would get to where we were-on the family ranch in remote SW Texas-during the Cuban missile crisis, and deciding that was probably not likely to happen, but being locked and loaded anyway. My dad was in the military at the time, TDY someplace, but when it got insane, every service member went on alert.
Sounds like the zombies/walking dead are basically the new incarnation of those “desperate people” of the bomb shelter era.