Bad analogy. The KKK were primarily white nationalists, racists. That was the primary driver of their ideology. The "Christian" part of "white Christian" was merely buttressing the European heritage of the founders.

After all, they hated Protestants who were black or non-NW Europeans, and all Catholics, with similar fervor.

Conversely, radical Islam is -only- about religion, they don't care about national identity, only religious agreement with, or submission to, their particular read of Islam.