This is the whole "No True Scotsman" Fallacy.
No true Scotsman or
appeal to purity is an
informal fallacy in which one attempts to protect their
generalized statement from a falsifying
counterexample by excluding the counterexample improperly.
[1][2][3] Rather than abandoning the falsified universal generalization or providing evidence that would disqualify the falsifying counterexample, a slightly modified generalization is constructed ad-hoc to definitionally exclude the undesirable specific case and similar counterexamples by appeal to
rhetoric.
[4] This rhetoric takes the form of emotionally charged but nonsubstantive purity platitudes such as "true", "pure", "genuine", "authentic", "real", etc.
[2]
Philosophy professor
Bradley Dowden explains the fallacy as an "
ad hoc rescue" of a refuted generalization attempt.
[1] The following is a simplified rendition of the fallacy:
[5]
Person A: "No
Scotsman puts sugar on his
porridge."
Person B: "But my uncle Angus is a Scotsman and he puts sugar on his porridge."
Person A: "But no
true Scotsman puts sugar on his porridge."
en.wikipedia.org
A person might truly think they are a Vegan if they don't eat meat 99% of the time, and maybe had that steak once. Some people consider themselves Vegan if they still eat Dairy or seafood. See, you are appealing to purity.
As for Christians not murdering people, sweet evil Jesus, man, Christians have been murdering people since Constantine rose the cross over the Milvian Bridge.
View attachment 901801
Not to mention all the Crusades, Inquistions, witch-burnings, etc. that "Christians" carried out with the full blessing of the Church.