Due to the (not so) new update of 1.2, villagers breed with each over to keep the population growing. By the way, you do need 2 or more alive to do that. They breed with each other over time, dependent on the number of houses in the village. Every 5 minutes or so, villagers mate to replace their "loved ones".
Hope that helps you in your endeavours to kill them (I am assuming).
WORD OF WARNING : The iron golem protectors spawn when there are 15 villagers or more, as their guardians. These badass golems will try to kill you if you mess with them or their people with funny noses. It takes 15 hits with a diamond sword to kill, so they can be annoying. They throw you up in the air, but are quite slow.