I think that snake ate the rabbit's entire family when the rabbit was just a kit. He witnessed the slaughter and decided at that moment to devote his life to avenging their deaths. After five years (a lot, in rabbit years) of constant, grueling training in multiple combat disciplines, propelled by an amalgum of hatred, madness, and unbending will, he finally bested his masters and decided it was time to realize his destiny.
Now that the deed is done, he's discovered that he's forgotten what it means to live without anger. He has lost himself to vengence, and now he's but an empty shell. Before the depression completely overtakes him, he has an epiphany. There's a whole world out there full of snakes who've grown complacent; their reign of cruelty has gone unchallenged for generation after generation. Those countless rabbits who've lost everything in a sudden slitherly flash have been without a champion.
Until now.
I don't think this is the last we've seen of him.