Psychological coherency
While I explored some specifics from Hitler's life, I do so to illustrate a point, and I also think this is a topic that needs to be continually remapped to reality in order to remain legitimate.
There are a couple of core ways to break down the complexities of the human psyche to make sense of ethical inconsistencies.
First is to explore a sense of Self and Other. When an individual values self at the expense of others, then this can lead to arrogance which is akin to dismissal of others. In its most extreme cases this is expressedin the dehumanization of others.
The second is to determine how Self is defined. If Self includes a partner, pet, child, country, etc, then the individual may protect those within that circle of Self even if they have fully dismissed the rights of Other.
There is no Self or all is Self. When the entire construct of Self and Other are dismantled, it can lead to a distilled result. When we look deeply enough at the Self, we begin to see cause and effects, and that based on how we are comprised, we have the capacity to be anyone. There are other philosophies that break down this sense of Self and can lead to altruism, at least in theory.
I do think there is an inherent logic to equating the needs of Self as one instance of a human being with the needs of another person who is also a human being. If we exhibit an abhorrent level of cruelty towards one being, I think it does limit the ability for compassion towards any other. Edit: And vice versa. True, deep compassion towards even one limits our capacity for cruelty.