Butterflies are a Passive Mob. It is unknown if they drop anything upon death.
Upon 1.11.2 update, butterflies are now identified as an Aggresive Mob and killing one will count to give the player the Monster Hunter achievement. They inflict no damage but can occasionally bump into the player, pushing them slightly.
Butterflies will despawn once the player has reached enough distance for the chunks to unload. This can be prevented by using a Name Tag on the Butterfly.