He just shot the fat guy cause he knew he was gonna turn
There's no evidence to support that. I can just as easily maintain he did it to put him out of his misery.
The fact is, if Rick did know that Shane was gonna come back, then it strains credibility that he'd suddenly forget just because he was a bit upset about having to kill Shane. It's the most important thing he could know at this point because it makes any death a threat. Rick may be emotional, but he's not an idiot, especially when it comes to keeping his family safe, and by extension keeping himself alive to protect them.