There is no fixed rule in English grammar that says which word is good and which word is derogatory, the only reason behind our perception of these words seem to be, well, habit.
Why "negro" is an insult but "black" is ok? Why Philepines is ok with the word "Pinoy" while Japan get pissed when you call them "japs".
Yankee used to be a offensive word, but now americans just accept it.
Moving on to politics.
Marxism used to be a word other communists used to bash Marx's theories, and Karl himself refused to use it.
So the game goes like this:
Term A
It's current usage.
It's AH usage and the history behind it.
For instance.
Islamo-fascism
A neologism which draws an analogy between the ideological characteristics of specific Islamist movements and a broad range of ... fascism .
Could the word be coined earlier and used on some strong man in a particular Muslim country who copied Mussolini's ideologies and made the trains run on time in his country? That would make *islamofascism more popular in the west than other muslim ideologies.
Just try.