For people who want a fuller description about why Scheler's insights were thought to be novel (despite much historical language about "hardening one's heart" and "log in thine own eye" and so on), the Wikipedia article does a decent job at placing his ideas into the context of the time: https://en.wikipedia.org/wiki/Ressentiment_(Scheler)

For those who like claims of fact to be backed up by evidence, note that people have detected an inverse correlation between self-esteem and bigotry (for example, as described in https://www.sciencedaily.com/releases/2011/02/110223151945.htm). Also relevant is evidence that the mismatch between explicitly stated and implicitly measured self esteem is the biggest driver: https://journals.sagepub.com/doi/abs/10.1177/0146167204271580. Of course, this is far from confirming Scheler's whole thesis (or even any significant part of it), but it at least has aspects of plausibility.

However, it is worth noting that even transiently provoked tribal protectiveness can substantially alter value judgments (https://www.sciencedirect.com/science/article/abs/pii/S0022103120304388), which casts some doubt on the centrality of the psychology of "ressentiment" in producing a depravity of value, at least within society as a whole. Perhaps it is better explained simply as a reflexive reaction to a threat to one's tribe?

The idea that a "struggle against injustice" matters is somewhat at odds with research that uncovers similar levels of bias across political parties in the United States, despite one party nominally styling itself as supporting a struggle against injustice (https://journals.sagepub.com/doi/abs/10.1177/1745691617746796?journalCode=ppsa and with moral judgments, https://onlinelibrary.wiley.com/doi/full/10.1111/pops.12582). Then again, this is far from conclusive because ressentiment talks about a very specific type of moral bias which was not fully explored in these studies.

Finally, if one is interested in a modern psychological perspective on some of these topics, one would do well to read The righteous mind: Why good people are divided by politics and religion (Haidt, 2012).