[Yea, Glenn should have shot Merle immediately, but, that's an awfully tough decision to make, to kill someone you know, in cold blood like that. He reasonably thought with the two of them, he and Maggie had the situation in hand, very bad decision, but, you can't really blae him and call him an idiot for it
Sure you can; Rick knew Shane since they were young, but when Shane's murder plot was uncovered, Rick had enough and did not hesitate. Glenn did not have that kind of connection to Merle, and unlike Shane (who steadily turned into a threat), Merle was advesarial from the start.
There was no reason to trust him, and he should not rely on Maggie, as it would only keep her in a dangerous situation...and as it turned out, it was that kind of situation.