Beorn is an enigmatic and powerful character in J.R.R. Tolkeinâs novel, âThe Hobbit.â His is a man of peace as well as a fierce warrior. He enjoys entertaining stories, laughing whole-heartedly, and yet is serious about protecting the innocent and the good.

Beorn joined the elves, men and hobbits in the Battle of the Five Armies. When he fought through the crush of orcs and killed Bolg, their chieftan, he helped turn the tide of the battle to victory for Bilbo and his friends.

http://www.enotes.com/hobbit

To answer the question of "Why did Beorn kill Bolg?" is fairly simply. Beorn, though powerful and warrior-like, is peace-loving. He is protective of his animals, and although distrustful of men, has no wish to harm any creature. He lives in the woods, and has no greed for material objects. He is the anti-thesis of the Goblin personality and way of life. Therefore, as a leader himself, he must face their leader.

Beorn is a shape-shifter. A peaceful creature, he allows the band of adventurers to stay in his cabin. He helps turn the tide of the Battle of Five Armies when he appears and kills the goblin general. He also protects Gandalf and Bilbo on part of their journey home. It was Beorn as a bear who decisively killed the goblin leader Bolg.