The difference is definitely not known to be due to current environments' relative abundance.

Maybe you got that from the Wiki article on bonobos which includes a deceptive quote of a 2014 Nature article. What the article actually says is that bonobos have a genetic adaptation due to historic evolution in environments with abundance which either allows for phenotypic variation in response to current environment, or which has resulted in biological evolution to less violence as a general policy (which would indeed be "nature"). Personally I'm strongly inclined towards the latter hypothesis because it's the most parsimonious and fits observation better, ie the number of observed bonobo murders is near zero despite varying environments. If we find modern bonobos living in some scarcity who take up murder as a result, then I'll change my mind.

Chimps on the other hand are believed to be violent because it confers genetic fitness (as per that same Nature article), and things such as the age and sex of their victims is predictive suggesting they murder in order to eliminate competition. This is their nature in the sense that this behavior is significantly impacted by their genes.

I mean, I'm not arguing that it's all cultural, or even epigenetic. But claiming that humans have nature X because chimps have nature X is definitely cherry-picking data points given that bonobos are just as closely related to us and behave very differently.

Indeed, humans clearly have both tendencies in our evolutionary lineage. But I think the point is that humans build societies which enforce behavioral norms independently of what would emerge based on the “natural” environment. Both bonobos and chimpanzees are locked in their strategies for conflict resolution, which may not always be appropriate for the prevailing environmental conditions.