Come on guys, you miss the whole well constructed plot.
Bolvar Fordragon is the key element of the whole fight against Arthas. As a paladin he knows that true Retribution demands sacrifices.
I'll explain to you the secret master plan.
Bolvar Fordragon works together with Alexstrasza. In a secret council they decide, that there is only one way to get rid of Arthas. To start the conflict between Horde and Alliance again. Split up, both factions have to increase their strength and army and get strong to attack Icecrown Citadel.
Infiltrating Icecrown citadel was nearly impossible so Alexstrasza and Bolvar used the trick of: "OH NO I'M DYING, CAPTURE ME!", while being burnt with the Flame of Life to conserve and even increase Bolvar's power. Alexstrasza's amulet is in fact a communicator directly linked to Bolvar. Now Tirion Fordring enters the stage.
Fordring's task is to bring the Ashbringer to Icecrown Citadel. When he meets Bolvar inside, after mocking Arthas and playing theatre (OH NO I WILL RESIST), Fordring gives the Ashbringer to Bolvar, because damn as a Lavaman, burnt in the fire of the Dragonlady, tortured by Arthas, he looks much more like the Ashbringer then Tirion does. Imbued with the power of the Dragonfire, combined with insider knowledge, due to his infiltration and combined with the power of the Ashbringer, in addition to his true path of Retribution, he kicks Arthas' ass.
Easy to figure out.