You will need to explain Biblo’s motivation. The Arkenstone was very important to Thorin, and knowing this Bilbo gave it to the Elven king to end the fighting.
You can argue that Bilbo was right or wrong, but either way you need to explain why he chose to keep the gem from Thorin.
The Arkensone is described as “the Heart of the Mountain.” It was the fairest of all of the gems, and extremely important to the dwarves. It has sentimental value to Thorin, because Thrain believed that he garnered power from it and it was proof that his family’s rule would never end.
"The Arkenstone! The Arkenstone!" murmured Thorin in the dark, half dreaming with his chin upon his knees. "It was like a globe with a thousand facets; it shone like silver in the firelight, like water in the sun, like snow under the stars, like rain upon the Moon!" (ch 12)
Bilbo finds the stone in the dragon’s lair, and knows what it is instantly because “there could not be two such gems, even in so marvellous a hoard” (ch 13).
Bilbo decides not to tell Thorin or the other dwarves about the Arkenstone. He is afraid of what might happen if the dwarves know he is hiding it. Bilbo decides to give it to the Elven king in order to end the violence. The Elvin king is expressed, especially with what is likely to happen to Bilbo when Thorin finds out. Gandalf arrives, and tells Bilbo to keep his heart up. Biblo is puzzled, but goes to face Thorin.
Understandably, Thorin is angry when he finds out that Bilbo gave away his Arkenstone.
"By the beard of Durin! I wish I had Gandalf here! Curse him for his choice of you! May his beard wither! As for you I will throw you to the rocks!" he cried and lifted Bilbo in his arms. (ch 17)
Gandalf intervenes, and Bilbo stands up to Thorin. He says he took the stone as his fourteenth share, and wanted to end the fighting. Rather than be impressed that Bilbo gave away his portion of the treasure, Thorin feels betrayed.
Bilbo was brave and clever in what he chose to do. Yet Thorin did not see it that way. It is up to you to argue if Bilbo was right or wrong, but be sure to include evidence.