The gods had forged a world out of the primordial chaos and through their acts brought order and balance, yet the unstoppable and indiscernible web of fate was still woven and all beings were to be bound by it.
Thirst for knowledge and cunning in his missions to achieve it had bought Odin more learning and understanding than any other being in the cosmos. He learned of his fate and the knowledge of his fate also sealed it in divine irony. For it was his treatment of the Fenrir that would be the wolf's motivation for attack.
Perhaps as a creature more than half giant and born therefore a being largely of chaos; Fenrir would always have sooner or later been a force of destruction purely by his nature. Odin may indeed have been aware of this and taken the action, he believed at the time, gave him the ability to protect the order of things the longest. More time to fill his hall of Valhalla, ready for the final war of Ragnarök.
But there's something more to this and the manner of Odin's apparent demise hints at what that could be. Odin is swallowed whole by Fenrir, but before his death, he throws the spear Gungnir (which never misses and as such could likely have found the heart of the wolf) over the battle field to mark the death of a warrior and rides to his doom without seeing harm to Fenrir.
Is it possible then that Odin knew there was no other way? That his death would secure the best possible outcome? Was it acceptance of justice? A sacrifice to save others, or did he once again sacrifice himself to himself? Embracing the chaos, becoming a part of the very fibres of the cosmos.
Some think of Ragnarök as part of a universal cycle and perhaps after the destruction, once again life will come screaming into existence as elemental fire and ice meet.
For other tellings of the tale, there are survivors of the war and the recovered world, once the fire subsides and the waters recede, is one of pureness & fertility. Odin's son Baldur the shining god, most fair and good of all beings is free from Helheim to take his rightful place in Asgard amongst the children of Thor and the others.
Over fresh waters and golden fields an eagle shrieks a primordial sound, free from voice. It sees the new world that doesn't yet need a god of war..