By keeping her diagnosis a secret, Olivia was making a bet with herself. If Ethan still loved her, she'd turn her death into her revenge against him. Even when she was dying, she wanted to make sure that he carried the guilt with him forever.

However, if Ethan did not love her anymore, it would be pointless to inform him about her diagnosis. Not only would she humiliate herself, but it'd also give Marina a good laugh.

With that in mind, Olivia left the bathroom and went downstairs. Madam Burg