Katherine cried because she was frustrated and truly angered by what she found out, rather than being disappointed about learning what Deborah did. She couldn't say she was completely surprised, after all, she already suspected her aunt. However, without evidence or solid proof, there was nothing she could do then.
Thinking about the times when her grandfather fell ill, it seemed as though they all lined up to when she wasn't around to take care of him—almost as if whatever Deborah did, the latter did when Katherine wasn't there to catch her. That theory she formed in her head left a bitter taste in her mouth, especially when she recalled how she wouldn't have caught her aunt if she hadn't gone back to retrieve her phone.
Anyone could have seen what Deborah did, given her door wasn't thoroughly closed—which was quite stupid on her part. Her carelessness caused her evil plans' demise—and for that, Katherine was lucky.
