In the world of the show, survival, ruthlessness, and pragmatism do trump morality. That’s the point. It’s about what compromises you’re willing to make to hold onto your humanity and society.
How many people would still be alive if Rick had simply left Shane behind when they were dropping off Randall? Or left Randall behind to be devoured by walkers in the first place? Or if he’d simply shot the prisoners dead?
What has morality ever done for his group?
It was unquestionably wrong, morally-speaking, for the Governor and his cronies to murder the guardsmen. But, if the guardsmen were within driving distance of Woodbury (which they were), then they were a threat, however remote. If they had supplies and weapons that Woodbury needed (which they did), they were a target.
There was no real scenario in which, had they eventually found Woodbury, the Governor and his compatriots wouldn’t have been better off having killed them when they had the upper hand. He can’t even integrate one angry black woman with a sword into his community – how’s he supposed to deal with a (probably tight-knit) squad of heavily armed men?
What are the possible outcomes:
[ol]
[li]Do nothing, the Guardsmen never come into contact with Woodbury, and their weapons and equipment vanish from the region. - Wash[/li][li]Do nothing, the Guardsmen find Woodbury and somehow integrate into the population. The male-female imbalance of the settlement increases dramatically. There are a dozen more mouths to feed without any consumate increase in resources. - Wash[/li][li]Same as 2, but the Guardsmen form an uncivil minority within Woodbury and cause a conflict, resulting in them being killed. - Loss[/li][li]Same as 3, but the Guardsmen succeed, killing or at least deposing the Governor and his cronies. - Loss[/li][li]Do nothing, the Guardsmen eventually encounter someone else who kills them and takes their weapons, and then later threatens Woodbury. - Loss[/li][li]Do nothing, some other force shows up to Woodbury, which lacks any heavy weapons with which to repel them. - Loss[/li][li]Kill them, take their stuff. - Win, but morally repugnant.[/li][/ol]
That doesn’t make it right for the Governor to murder them, and I don’t defend the character’s actions, except to say that I understand them and am glad to see him portrayed as a believably human villain, who thinks that what he is doing is justified, if not necessarily righteous, as opposed to a simple thug, which would be boring.