Orcs: Characteristics (p4)

Wickedness and violence were their nature, and it was not uncommon for them to kill each other in petty quarrels. They had a destructive effect on nature, especially forests, and often destroyed trees to fuel their war-making, particularly those near Isengard. Orcs had unchangeable disdain for all Elves, Dwarves, and Men, though they fought alongside wicked men under Morgoth and then Sauron in the First and Third Age and were also said to have made alliances with wicked dwarves.