They have already explored that to a degree: Tyresee refused to kill Martin--who was the person who knocked out Bob in the woods. When the Terminus gang were killed at the church, Tyreese observed from a doorway and we can assume he saw Martin (being killed by his sister), knowing that he had Martin's life in his hands, but let him live.
I wouldn't say they explored that. I'd call it a flirtation. They dangled it out there & then sidestepped it & got Tyrese off the hook. He lied to Carol. Betrayed their trust in a small way, & was afraid to let it be known that he couldn't or wouldn't do it. That's bad form in a tribe
Again, to be clear, defense is not in question, but absolute brutality (hacking a man to death, instead--for example--of a simple gunshot) is adopting an excessively violent way of being that would be at home at Woodbury (ex. beheading the helicopter pilot to be part of a psychotic display)--the very type of institution Rick once tried to separate himself (and the group) from.
He told the guy that he only got 2 minutes more life because he wasn't worth wasting a bullet on. They finished that threat with 2 bullets & will, & the hesitation to do it earlier cost them someone (Even though Bob was doomed regardless). If it hadn't been Bob, it would've been one of the others, because they were not considering the retaliation
I'm not advocating 100% mercilessness, but in an anarchy, mercy is as rare a commodity as trust. They clearly have demonstrated a policy for entrusting someone. They've even got a little interview process. They must also instill as definitive a policy & practice for determining the appropriate time for mercy or mercilessness, and when bullets are gold, barbarism is free. They are all in a squad now, a squad who has & will face combat. They must act accordingly
If people who are planning to gut you & chow down on your carcass don't qualify for some measure of mercilessness, then no one will, & you are allowing your principles to make you & yours expendable. So yeah, there's a hardening of spirit that comes with that turf. The choice is that you can either come to grips with being hardened toward enemies, or you can grow hardened & wearied from losing those you trust & care for, because you don't take the appropriate measures & apply the proper attitudes.
Comparing that to the Governor collecting skulls is a weak analogy.
What--exactly--are you living for when you have effectively incorporated the tools of your enemies?
Incorporating the tools of your enemies is totally necessary. Incorporating their philosophies is optional. Gareth & his people chose the latter, & it led them to ruin. Rick is not advocating a philosophy of be the butcher or the cattle. He is not self serving to the point that he executes people who may weaken his leadership role, like the Governor did to that military squad