Before "Night of the Living Dead", zombies behaved in a variety of different
ways. Romero's film had such an impact, though, that firm rules of zombie behavior
developed afterwards. Although these rules are not obeyed explicitly in every film, some
hard-core zombie film lovers get upset if they are not.
The only way to stop a zombie is to destroy the brain or disconnect it from the rest of the body. This is preferably done with a shotgun blast to the head.
If you are killed by a zombie, you become one yourself. This is either because everyone
dead is becoming a zombie, or because being a zombie is some kind of communicable disease,
and if you die after being bitten by one, then you become one.
Any attempt to bring anyone back to life for the purposes of
science
love
power
vengeance
alleviating boredom
will ultimately go awry, resulting in at least one rampaging, mindless undead person.
So don't say I didn't warn you.