The scheming ATF agent was brutally killed by Opie (Ryan Hurst) in last night's third season finale.
"The bad guy must go down," Walker told TV Guide. "Maybe [Stahl] was on the side of right, but who she was as a human being was very vile."
Walker suggested that Stahl's fate had been sealed from the moment she struck a deal with biker Jax (Charlie Hunnam).
"Once Stahl and Jax were in business, it was kind of over, and I knew that," she revealed. "She's such a bad, bad person that it's really hard to go anywhere else with her."
She continued: "Jax had proven how smart he was to her in the interview in the jail in season two... and I think she was aware he could turn on her. She just couldn't see how."
Sons of Anarchy will return to FX for a fourth season in 2011.