The Kinslaying was the first killing of Elves by other Elves.

The followers of Fëanor attacked the Teleri in Alqualondë when they wished to leave Valinor. The Noldor attacked because the Teleri wouldn't let the Noldor use some of their swan boats to sail to Middle-earth.

The Teleri withstood the Noldor attack for some time, and the fighting went on on the ships and the quays. The people of Fëanor were driven back three times, with grevious losses to each side. Most of the Teleri were slain eventually, because the Noldor were fierce and desperate, and had swords and armour, while the Teleri had slender bows.

This crime was to cause strife throughout Elvish history.
Encyclopedia entry originally written by Tinuviel_1