At the beginning Prof. Nicholson told Cahill that he had told Mrs Nicholson about Cahill's son stealing the work. So Cahill would need to kill her straight away also, she knew the secret and would be even more of a potential threat when her husband has just been killed after he said he would reveal that secret. But Cahill did not kill her.
I THINK Cahill was counting on her not being able to reveal the secret because she was acting as his son’s therapist. Still, it was a big chance to take. And as we saw, the son eventually admitted that it wasn’t his work anyway. The whole murder was useless.