The presumption, both by the op and by those who invoke “mental illness” as an explanation, is that an individual’s belief systems grow out of some internally consistent logic, rather than that most of us develop our beliefs for a host of irrational reasons (personal experiences, cultural norms within our family of origin and the community in which we had spent our formative years, the natural tendency of humans to develop preferences to kinship and fictive kinship groups such as those of tribalism and nationalism …)
That presumption does not IMHO reflect reality. More often beliefs come first and then come the as-if stories of “logic” to justify the beliefs.
Anyway, playing the logic game, there is no conflict:
Belief A: Nazis were bad for a host of reasons. Sure a racist and nationalist agenda (the two overlapped) was the major identifying feature.
Belief B: His self identified group is better than other groups and groups should keep to themselves with severe consequences for failure to do so.
There were a large set of beliefs Nazis had, and in isolation he likely agreed with many of them. Many highly placed in the American government of the era agreed with the statement there was “a Jewish problem” that needed to be fixed even if they did not agree with genocide as the solution and that Whites were racially superior to Blacks. Friends of his died fighting the Nazis (and they did not do so because the Nazis were racist); to see any Americans adopting Nazism dishonors their memory to him.
No opposing beliefs really.