Quick answer. She was bat crazy. Longer answer. Her desire to kill the witnesses who could reveal her father's identity warred with her growing love for Kirk.
Much longer answer: Lenore killed those witnesses when she thought they had realized who were father was. After all, she probably had many opportunities to kill Leighton before she did but she did so only after he was convinced that Karidian was Kodos. Perhaps she then jumped the gun so to speak on Kevin Riley but there it is.
Bottom line. She was bat crazy. Note, consider that Lenore wasn't only murderously crazy but she was also able to apparently completely conceal her madness and her knowledge from her father.