as salam alaikum
if a person is openly a sinner, unjust or oppress other people, he is considered a wrongdoer (zalim).
If a person consciously, willingly and openly fights the truth, while knowing that it is the truth, it is disbelief (kufr) or apostasy (irtidad).
If a person reveals what a Muslim person hide from his sins, it is called backbiting (ghibah).
So the basic difference is that declaration of zulm, kufr and apostasy are related to open attitudes and behaviors while backbiting is related to something that a Muslim person wish to conceal about himself.
As far as the Khawarij, their enmity against Ali, peace be upon him, has been seen as an aversion for the truth even if it is not improbable that among their ranks were also unaware people misled by their leaders.
With prayers for your success.