"The most direct and simplest way to put an end to a zombie is in hand-to-hand combat. Every weapon is a little different, but it always comes down to the same thing: remove the head or destroy the brain."
While all melee weapons have a chance to instant-kill, most require multiple strikes in order to finish a single zombie. Therefore, it is recommended that the player uses the knockdown-then-execute combo instead of mashing the attack button to conserve weapon durability.
Initially, melee weapons are only good for single encounters; any more than 3 zombies will pose quite a threat to inexperienced survivors. However, when survivors specialize in a melee weapon, eventually they can choose a special attack that can knockdown/ incapacitate multiple zombies with one hit, which vastly improves the capability of dealing with hordes single-handedly.