The last chosen hero of Akatosh as well as the savior and champion of humanity would've fallen for greed long before if that was his motive (In my eyes, anyways). He's going to need to go through something pretty life altering or corrupting. reading the Black Books plants the seed, then Mora, realizing what's happening, and foreseeing the betrayal, attempts to rein in the dragonborn. Possibly killing someone or many important to him?