WHY?

Sara killed Becky because Becky kept abusing her son (Christopher). When Becky came in and scolded the boy for writing on the table and started shaking him and ultimately threw him down it was the last straw for Sara. Sara couldn't take watching him being abused anymore and so she killed her. Sara was especially sensitive to this, not only because she herself really loved Christopher, but also because she wanted a child of her own and couldn't conceive. She viewed Christopher as her own child and she protected him just as a mother would protect her own child.