Jane Austen?
Are you kidding me? We're talking wizards and betrayal and murder and you bring up Jane Austen? No offense, but Jane Austen has little to do with Snape and his evildoings.
First, let's explore Snape's motivation. He has accomplished much without the help of Voldemort and the support of Dumbledore. He has achieved great prestige and respect as a Hogwarts professor. But there are two things he has yet to achieve: power and immortality.
He is motivated by both. He makes no secret that he wants to be the Defesne Against the Dark Arts professor, a position of greater power than potions professor, and one that is believed to be cursed. Either Snape has the arrogance to believe he can brek the curse or he believes becoming the Defese Against the Dark Arts professor will lead to greater respect and prestige among the wizard world.
But his ultimate motivation is self-preseveration. As a member of the House of Slytherin, this is an inherent characteristic. All Slytherins fear death. Snape is no different. He will do anything, including murder, to secure his own existence. He has no loyalties, save unto himself, and fears death more than Voldemort or Dumbledore.
Snape is still evil. And I am beginning to think so is Jane Austen.