This discussion may contain spoilers!
Let's say Christa comes back in E5, Clementine can see her in the distance and Christa can see her and they both run up to eachother to hug (I know it's Cliche just bear with me) Just than Nick aims his gun and shoots Christa in the Chest and she dies shortly after. When he sees Clementine is crying he says he didn't mean to. Clementine pulls out her gun and shoots Nick (choice) and he dies right there. It'd be crazy if that's how things play out if you kept Nick alive because the story just repeated itself in E1 when Michelle killed Omid and Christa killed Michelle. If you didn't keep Nick alive Christa dies later on (That'd be really cool how we're pretty much in Christa's place from 16 months ago when Omid died. & don't get offended just because I said Christa and Nick died, I'm a fan of them to but I just want to hear what you'd do.)
And here is a very beautiful photo of Christa herself.