Religion

Mostly those who assume that members of their tribe are to be supported and justified.

That makes it a human thing: We exculpate our community, finding reasons to justify what they do, minimizing the evidence against them, and giving them the benefit of the doubt when possible; and treat outsiders as enemies, finding reasons to make anything they do bad, interpreting the evidence in the harshest possible light, and denying them any benefit of the doubt when possible. Criticial thinking's purpose is to fight cognitive biases. This is a cognitive bias.