I still think the absolute best story in the game is the Thorn / Last Word Grimoire.
A young kid seeing a dazzling Guardian, in awe runs up to him and holds his gun, The Last Word... realizing this path he wants to set on.
A man corrupted by the Darkness so fully that he forgets his past self, abandons his heroic storied life as a Guardian and assumes a new identity; his old Rose becoming a Weapon of Sorrow, the Thorn.
His first and last friend confronting him, urging him to recall who he was, while the carcass of that former guardian saying that the hero of old was no longer, and only he and his darkness prevailed.
Imagine how freaking cool it would have been to finish up the Thorn bounty in an Elimination map, riddled with dialogue, against a bot using Thorn. Defeat this old guardian to claim his weapon. No Special, no Heavy. Just 5 rounds of PvE in a PvP format with dialogue, lore, and a great reward.
It's painful how much potential this game's back story has.