I don't read the comics, but was the phone-call a significant moment in the comics?
A lot of guys were saying both Glenn and Michonne should have killed Merle. That is what we want them to do, but is it what their characters would have done?
I'm not sure we were given a lot of background info on Glenn, but I think he was just an average guy. Would killing Merle (before he jumped Maggie) right then and there have made sense in the context of his character? Merle had already lowered his weapon and Glenn was in the process of trying to negotiate with him in trying to have Daryl meet him at that parking lot. That seemed like something Glenn would have done, plus he probably didn't want to have to deal with Daryl as the guy who killed his brother.
As for Michonne, like they mentioned in the Talking Dead, it's not in her character to do so. She's (from what I've seen on the show) not as instinctive as some of the other guys. She sits back and evaluates the situation first before making her move. Furthermore, she had no idea who Glenn and Maggie were. Would it have been in her best-interests to risk her life (she didn't have a gun) for these people?
I would prefer it if they didn't kill-off Daryl!
I wouldn't have been easy, but she was holding the red shopping basket (with the baby formula), so that could have stood out to Rick.