There is also a difference between Officially Accepted History and reality.
The Minbari party line might be no minbari has killed another minbari for a thousand years; and exceptions to this are not talked about, or covered up.
Its a very rigid, caste based system- if people are told something is true, very few people are going to publically doubt it. Conformaty, and appearances are everything.
Check out Feudal Japan, or even modern Japan...
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A ranger operating away from high command finds a Minbari working for the Shadows. His only options are to kill him, or let him escape.
Ranger kills him. Do you think he is going to buy advertising space in Minbari Daily with a big banner saying I Minabari ranger have killed a minbari from warrior caste X?
He will not lie about, but not stating certain information, or presenting in an incomplete truth that leads to a different conclusion is consistent with the show. Delen lies by ommission pratically every thing she opens her mouth in the first season 8)
The ranger could say "He died by an ancient evil;" "He had an accident;" "I don't think he will be returning to Minbari space;" "He is seeking spirtual enlightment;" "He went to the ocean"
All are true... and misleading.