German hospitals were filled with soldiers who went insane due to having been made to commit war crimes in WW2, and they suffered greatly, reliving the horrors they had inflicted on others. A healthy mind is not a thing, but a dynamic process, and that process can be disrupted and/or completely crash due to the impact of negative kamma (however, this should in no way be interpreted to mean that all mental suffering of this kind is the result of kamma).
While humans are certainly capable of inflicting hellish suffering on others, I also suspect that any hellish afterlife is a mental nightmare conditioned by negative actions. So in my opinion, the actions are not negative or positive because they are natural or unnatural in some philosophical system, or because they have an inherent meaning (whatever that is supposed to mean), or because some allegedly divine being likes or dislikes them.
In fact, I have yet to met any proponent of «moral natural law» who consistently applies it to their own life. So for instance, a minister who argues that sexual reproductive organs have been designed with a specific purpose in mind, and using them in any other way is sinful, may do so while adding cream to their coffee. The irony being that they are drinking cow’s milk, «designed» (by evolution) to strenghten the immune system of a calf, taken from a teat «designed» to be inserted into the mouth of the calf, not an «unnatural» milking machine. Such examples can be multiplied effortlessly.
According to the Dhamma as I understand it, actions that harm mental and spiritual health are negative because they lead to harm, and actions that improve mental and spiritual health are positive because of the impact they have, not just externally in the world, but internally in one’s mental continuum.
In my opinion, Buddhist ethics are deeply rational, and every bit as applicable in the modern world as in ancient times because of this.