Lava

Carefully place a bucket of lava by your victim innocent bystander. Once they are aflame, pick it up again so it doesn't spread too far!

Fire

Light them on fire with flint & steel. You can punch out the flames immediately to minimize collateral damage.

Suffocation

Carefully dig a hole directly beneath them, and fill it in with sand/gravel.

Drowning

Construct a water flow that drowns them (look up some mob traps). 'Accidentally' knock them into it.

Zombie Apocalypse

Got access to a zombie spawner? 'Pipe' your zombies with water flows into the center of the village. Unless they've got an iron golem, there may be a few casualties.

Remember: It's not you, it's the hostile environment.