Here's what I think happened:
Dale (played by Jeffrey DeMunn) was a victim of backroom politics on the set. DeMunn had worked with Darabont before on "The Mist," and Darabont was the one that got him hired to play a part on "The Walking Dead." When Darabont was fired over a financial dispute, DeMunn asked for his acting contract to be released, so that he could quit the show.
DeMunn had since changed his mind, but it was too late. His contract had been released, Dale's death had been written and the funding for the zombie effects had been slated. So in effect, he had already been written off the show and there was no backing out of it. Thus, Dale was killed by the weakest stealth zombie in history; the scrawny bearded swamp walker.
What do you think?