Scarper is a very lightweight Forge mod that changes the way passive mobs behave when they witness a player attacking other mobs.
With this mod installed, if you attack a passive mob, other passive mobs within a configurable distance will run away from the player if the player attempts to get too close.
The mod can be configured to allow only cause the same type of passive mob to scarper from the player, or all types of passive mobs.
There's also an option that causes any hostile mobs within range to hunt you down.