Gjallarhorn
N'yog-Sothep
I can't give you an honest that answer to that, I think that modern society has shaped the psyche of modern man to such a degree that it's almost impossible to compare pre modern man to modern man. I think we all still have what's called a conscience, which is your innate ability to tell right from wrong. if that is still healthy, as in intact and fully functioning, if one still feels guilt for ones wrongs, then that is, at least for the most part, mentally healthy. In other words if you commit a selfish act, which is harmful to another, and don't feel bad about it, you might not be a very healthy individual mentally.
Why would "pre-modern man" be any better mentally than now, considering the archaic laws they enacted in the first place?
You give no method of determining. You simply don't like society.