From what I understand, the guy who invented the Landru computer unilaterally decided for his people that they needed to return to a simpler time, and built Landru to enforce that. Over the millennia, the society devolved and forgot its origins or why they lived the way they did. Seems to me when a computer designed by a tyrant who has been dead for 6000 years is deciding for an entire society and using murder as a means to control it, and that society doesn't even know why, then that is a stunted society that is being controlled by unnatural progression, and has devolved. That was probably the rationalization Kirk made when he destroyed Landru. Destroying Landru wasn't going to hurt that society, because there was no where to go but up.