Why Nietszche acts like a dick
My interpretation goes back to the Eternal Return. I see this in part as a test for the Superman. The author speaks of wanting all of the past and present to repeat indefinitely so that one's actions could repeat indefinitely. I think "Superman" means roughly someone who would feel this way all the time. I also think Celia "Crazy Woman" Green has a big part of the picture: "it is plain that the idea is connected with the existential perception that the events of your life really exist." We might add that according to the evidence, every action or event really involves a host of others back into the distant past. So thinking about the Eternal Return easily shades into thinking about reality.
Nietzsche doesn't attack people's motives because feeling some emotion automatically makes you wrong. He attacks motives that the writers in question do not want to recognize in themselves, because refusing to look at the source of one's actions prevents one from contemplating reality. You can't wholeheartedly affirm your actions, history and will if you deny your history and will. It hinders the creation of the Übermensch, who would provide a conscious purpose for reality. A religion of envy in particular would rule this out because in Nietzsche's view the worshipers could never accept the true origins of their slave morality. ("Slave morality" sounds like an insult precisely because nobody's 'moral' feelings meet the definition fully. Everyone feels the influence of some noble morality that calls it shameful to live in slavery or to think like a slave.) Nietzsche likes "nobility" not only because it creates good opportunities for dickishness, but also because he seems to consider it necessary for facing reality with joy.