When you hit a zombie with a sword that's been enchanted with Smite, it temporarily disables the extra spawning powers of the zombie (normally zombies that are hurt have a chance of spawning additional zombies nearby). Smite is a holy, anti-undead enchantment, designed for fighting against evil and darkness.
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I've almost never once been killed by a zombie; and the few times I had were in times which I was completely surrounded and or already starved. Zombies are weak and easy to kill. The spawning chance isn't so high that zombies are overpowered, I prefer to think of it as zombies being "attracted" to the commotion of a fight.
I don't really think this is a good idea because zombies only spawn more zombies when you attack them. With a sword with smite, they die in fewer hits, giving them less of a chance to spawn more zombies anyway.
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