You know, another reason Kay may have had the abortion is to show how the evil of the Corleones has infected her family. Before she married Michael, she was a good girl -- a simple teacher with typical WASPish dreams of a loving husband with a legitimate career and a family. She could go as she pleased if she needed to grocery shop, meet other wives to chat, or take her children out to an outing somewhere.
After marrying Michael, the "evil" of his profession ruins those dreams. After the shooting attempt in Part II, she can't even leave the compound to shop or go out with her kids. She even comments how she's kept a prisoner. At this point, the evil of Michael and his profession has infected her as well and she feels that if killing is justified for him to do to keep the life he leads, then it is justified for her as well to get to a life she wants to lead.
