If an ideology or belief system or the like is only attested in the historical record by those who opposed it (who thus had every reason to misrepresent it), perhaps because none of its own adherents' writings survive, how do historians treat this information to understand underlying historical realities?
Well, if there are no records for the "victim", we usually turn to the archeological records. It's very hard to completely wipe out ones history, this question might work in a vacuum but one culture usually traded and interacted with many other cultures so eventually we would find different written perspectives, from both aggressors, allies and neutral parties to create a historical narrative.
Also remember that history is written according to the historian ideology that can also change the way we see each culture for example, a Marxist Historian would focus on the class struggle of the culture while a Liberal would focus on material advances. Not to mention that the field of history is divided into 3 specializations "Economy", "Culture" and "Political" and depending on the culture, a historian with specialization X might have a lot more meat on his plate than a historian with specialization Y
In the end, history is always someone's opinion, be it from the cultures involved or the historian that is working on them.
PS: I'm a Brasilian History Bachelor and I don't know if the Historical Theory changes a lot from country to country
The Cathars are an interesting such example, with doubt on whether they existed at all!
/u/idjet : https://www.reddit.com/r/AskHistorians/comments/20qcoq/cathars_and_ranters_didnt_exist/