I think Merle realized how grave his situation was once they left him on the roof. He was only left there because they had to leave in a hurry and Rick would have never left him there on purpose anyway. I'd also guess that Rick would have left Merle there had Lori not asked him to tell Daryl where he was...Rick then realized he not only had to tell he was honor bound to go and get him himself.
I have to give props to Rick though, I would not have cared if Merle died, he was an ass and the new world doesn't need people with that attitude. Heck one of the first things Merle says to the group is how ironic it was that Michonne had two African Americans chain ganged to her - who herself was possibly African American. Merle is an ass, I'm surprised he didn't kill anyone prior to the zombies.
I don't recall Rick ever asking Merle do do it anyway - he says he did, but I figured that was him taking responsibility for something wrong happening and not telling the others the truth like he seems to do (often). Merle does it because he knows Rick really wont, and I'm honestly surprised Rick would even think of it after what happens with Gleen and Meggs. Herschel should have said he would not trade her in to save his daughters out of all of them I expect him to be the moral center of the group with Dale gone.