This discussion may contain spoilers!
How Walter decides Nick's fate... I noticed something oddly familiar about it when I first heard about it, then I realised why- it was EXACTLY how I treated Ben's fate in my first two Season 1 playthroughs.
In my first S1Ep4 playthrough, Clem was at the house, I voted to kick Ben out, and I was so mad at him I refused to kill the Crawford walker attacking him on the bell and just let him drop right there. Then during my next playthrough, I brought Clem with me, and I was going to drop Ben again until I heard Clem saying that Ben was her friend, which convinced me to shoot the walker and save Ben.
Which is just like how if Clem doesn't say that Nick's a good guy, Walter will refuse to kill the walker attacking him because he's mad at him, and if she does say he's a good guy, he will reluctantly save him.