. >> former marine and firearms expert, weldon mcdavid, was working that night. >> a customer walked i could tell something was wrong with her. i couldn't tell exactly what but i could read she was under some kind of stress. >> he was, yeah, can i help you? i said, i need to buy a gun. it's not just for putting it away. my life is in danger. it's from my husband. he was like, oh. >> at that point, i asked her, are you prepared to kill him? because that is a very real possibility. if it's his life, your son's life or your life, then you have to make that decision. and after a few moments, she said, yes, if i have to. >> weldon sold her a gun, a .45 caliber pistol. but there was a 10 day waiting period before she could bring it home. 10 days before she'd have her weapon and could put her plan to leave sean in motion. >> i wasn't going to do anything until i had the gun in my hand. >>> coming up. will all that careful planning be for nothing? crystal becomes terrified that her last night at home with sean may be her last night on earth. >> i thought, you know, i'm going to die if i leav