totally agree, I'd even go so far as to say that Herschel totally misread what was going on with the kid Carl shot. He was seeing a teenage kid in the old world. Not a potential enemy in the real world.
But that's the point: Herschel saw the truth: the teenager was not some warrior, and he sure as hell was not Martinez or the Bowman. He was a kid on the run from an overwhelming situation he would never be prepared for--forced to be a soldier. He was running for his life, and Carl murdered him thanks to the attitude he reinforced to Rick later in the episode.
He believes killing to avoid being killed is the one and only mindset for the zombie world, despite examples of that behavior--played out in front of his face over the course of a year--failed. The Governor is a great example: he sent Merle to kill Michonne, but ended up kidnapping Maggie and Glen--setting the Woodbury raid (and multiple deaths) in motion.
If the Governor left Michonne alone, then no Merle to kidnap the couple, no raid, no deaths.
Shane murdered Randall in order to lure Rick into a death trap--for some moronic idea that only he was good for Lori and Carl, and how did that morally bankrupt, ends justifies the means ideology serve him?
The world changed, but if humanity is lost, then what is the point of continuing?
Embracing a nihilistic worldview is unnatural for a functioning, peaceful society, and Rick's group do seek to survive in peace, even while protecting themselves from threats.