"but I can’t hold that belief without irrefutable evidence "

That's a thought after my own xing. Well considered. And, In the case of having irrefutable evidence, belief is superfluous in a sense, since perception of evidence is prior to belief and sufficient in itself.

The concept of 'human nature' is a rather odd category, also seeming to me to be without utility because anything human beings do could be considered 'human nature'. So that doesn't allow us to understand or distinguish. It doesn't tell us anything new, and it postulates itself as existing prior to observing that their is such a thing. The concept is often used as a sort-of 'wastebasket' category and people throw anything they don't want to consider into it as a way of explaining away certain behaviors. You ever notice that? Though i'm not implying you are doing that here.

"but humans are bit too clever and our conditioning and learning and intellect and so on get in the way."

The nature of believing in the concept of being a separate independent self is to bolster that 'self' by any means necessary, the violence of a psychopath is a tactic to do that. Cleverness is another tactic. Seeking reward, recognition, and attention are other tactics. Most of us use tactics like that for that same purpose in our own ways, whether or not we know we are doing it. We're trying to protect the "little independent me" from the big bad everything else we've assumed is separate from us. Those tactics are covering up the flow of being. Part of the nature of being is acceptance and flow, or 'wu wei'. We don't learn that. We express it on the heels of perceiving and then unlearning the tactics and patterns that prevent it. 'Violence' can be included in that flow, the difference is whether that violence is coming from a conditioned and insecure belief in 'independence', or coming as an expression of the flow of being. And there is plenty of counter evidence to the nature of human beings as violent. Many people have lived their lives bettering themselves, and their neighbors, and ecosystems, never 'taking up arms' for personal gain. It's just that such acts don't as frequently make headlines, or if they do, the dramas of conflict seem to outweigh them because a 'little me' can bolster better in drama than harmony. A little me remembers that better, and creates it more. Whereas a sense of personal harmony stabilizes in absence of that belief.

"My understanding of wu-wei has built in the concept of causes/consequences."

Can you elaborate on that further? i'd like to hear more about what you mean...