http://minecraft.gamepedia.com/Village

A player's popularity starts at zero, and ranges between -30 and 10, and the following can alter a player's popularity:

Popularity of Actions

Action Popularity Change

Trading a villager for the last offer slot on their list +1

Attacking a villager -1

Killing a villager -2

Attacking a villager child -3

Killing a village's Iron Golem -5

A player's popularity does not reset on death, and players cannot alter other players' popularity. Popularity changes only happen once, so if you attacked a villager, then brought him to a different village, you would get the -1 popularity in the first village, but not the second. Popularity is stored per village; a player may have a high popularity in one village and a very low one in another. When a player acts directly on a villager, particles around that villager will indicate the change in popularity. Conversely, because popularity is stored per village, if the entire village is destroyed, any accumulated popularity, positive or negative, is also eliminated.

If a player has -15 popularity or less, iron golems of that village will become aggressive to that player. If an iron golem is idle, it may become aggressive to the nearest player with -15 or lower popularity. However, "nearest" can be any distance at all, so if the village's chunks are loaded (perhaps by another player), the golems can turn hostile even after the unpopular player has traveled across the world.

If a villager dies to a non-mob, non-player source while a player is within 16 blocks, or if a monster kills a villager, then no villager in the village will mate for approximately 3 minutes.