They have a semblance of society here, no matter how much bad writing tries to make a reasoned group decision thought out over several days into a crazy murderous Shane-mob. Dale threw a hissy fit because it wasn't the society he was used to. He could not accept the possibility that this was a valid course of action, even though the others actually listened to him and tried to see it from his point of view.
First off, everyone in the group agreed with what you're saying--including Rick--except for Dale and Andrea, the latter of which didn't care enough to try and stop the execution. In the end, Rick didn't go through with the execution because he had an emotional reaction to the way Carl was behaving. The group did not then, as a unit, adopt Dale's viewpoint and turn to the camera to moralize to the audience. They remained silent, and we don't know what any of them thought about what happened.
Dale was trying to keep his group from turning into Randall's group: people with so little regard for "the other" that they abandon morality--consideration of each situation as perhaps more than black or white--for an existence of animal instinct. Even if the planned execution of the prisoner was more symbolic than it was a literal turn down a slippery slope to savagery, Dale was right. There is no way to pinpoint what exactly would turn them all into inhuman monsters, but executing someone who
might not pose a threat because it's easier and safer is a pretty damn good guess.
(And come on, Dale was the
least ego-driven character in the group, by far. Maybe the delivery of his argument was shrill and annoying, but that's got nothing to do with the content of that argument.)
In contrast, Shane
is murderous. He tries to paint his behavior and way of doing things as motivated by rational thought andconcern for the group's survival, but he's just making excuses for himself to be aggressive. (Like letting the zombies out of the barn so he can be
a real mayun with gunz blazin' and expose everyone else to danger in the process.) He's a literal narcissist: he can't handle rejection, he can't stand not being top dog, he views other people as props to manipulate for his own benefit (see: Otis), and he gets angry and violent when people second-guess his "obviously correct" suggestions. He is not simply a reasoned man trying to survive. Once again, Dale was right: Shane belongs in the show's world as it has become because Shane is just a killer looking for excuses to kill.
"Civilized" societies have done far worse than execute attempted murderers in life-or-death survival situations.
Until they tortured him and gave him a real reason to run back to the group that abandoned him for dead, Randall was not a definite threat. Living in a world that has an undeniable "survival of the fittest" reality underlying everything does not make every single situation into a"life or death and oh my god attack with deadly force before thinking" situation. (Example: The real world.)
Rick and the rest of the group want some semblance of society back. How do they get from where they are to where they want to be by using a bullet to "solve" all
possibly dangerous situations while ignoring their own morality? They don't. Like one of the characters said in an earlier episode, the world had always been a dangerous place with the only assurance being death. What their world has become doesn't mean they have to, or should, compromise the values that make them more than mere frightened animals. (Again, I urge anyone who hasn't to read or see the movie version of
The Road, which does a much better job exploring these themes than TWD does.)
ALL OF THAT SAID: this show has clear examples of poor writing, and I would never argue against that. Heh.
tl;dr version:
1) AMC fires Frank Darabont
2) Jeffrey DeMunn (Dale) wants off the show cause his close friend was fired
3) Then he changes his mind, wants to stay
4) Writers kill him anyway
Ugh... very disappointing :/ I wonder sometimes if AMC has any idea what the fuck they're doing. They seem to be thriving despite their best efforts to fail.