Reluctant Killer

A policeman once told Katherine that anyone is capable of killing another person under the right circumstances. She didn't believe that she could ever someone's life, until her husband Jacob woke her in the middle of the night with the whispered words, "They've found me." Before she could ask who, Jacob handed her his loaded handgun and said, "If any of them get past me, you need to take care of him. She thought she'd never be able to shoot a person. Yet here she was, gun still hot in her hand, an intruder covered in blood at her feet.
Who was this man in her closet?
More Importantly, who was Jacob?