Aggression is usually portrayed to the invading side's public as defensive (preemptive strike, defence of minorities, official request for intervention, etc.) in nature. Why was there a need for the establishment of a "protectorate" in ethnically already purely czech lands in the rhetoric of the nazi leadership?
"Ethnically already purely Czech lands" is where you went brutally wrong.
So let's back up. The border regions of Czechoslovakia with Germany before the war were mostly German. Bohemia and Moravia as a whole may habe been 30% to almost 50% German or more in cities. But in the border areas like the Bohemian forest or the Sudete it was more like 90% German.
But even in the capital of Prague, street signs were still in two languages, etc. after being Austrian for 300 years.
The Nazis simply made up and then complained about (or overly exaggerated any real occurrence of) discrimination of Germans in Czechoslovakia.
Add onto this the overwhelming Czech-Germans support of the Nazis, and Hitler pretended to have a bargaining chip when negotiating with France/UK ("Czechs are committing atrocities to their German-speaking citizens!!" -- total BS).
While not true, still the right-wing newspapers were reporting almost daily "atrocities" against Germans in Czech lands.
The needed to "protect" the "Volksdeutsche" i.e. germans living outside of Germany's borders (that's the Nazi view).
Right after WWII Czechs actually did commit some atrocities and war crimes by kicking all germans out -- Czech-German or other. Often with just the clothes on their backs and brought them to the border: harsh mountainous area in the winter. --but that time no one cared anymore.