I'm not saying she could have used spousal abuse as a defense at trial; rather, she could have, and any competent defense attorney would have, claimed that it was a crime of passion at the sentencing hearing. That would almost certainly enable her to avoid the death penalty. Indeed, it would have been very unlikely for her to get convicted of premeditated murder in the first place, because the prosecution would have had to prove that she planned for Sam to come home just as she was leaving with Steve. Without proof of premeditation, she would probably only have been convicted of murder two or even manslaughter (exact statutes vary by state and year), which would preclude the death penalty.