But it doesn’t matter that you weren’t there. Based on the character of the statement—that an entire ethnic group is responsible for the death of one man—we know it can’t be true.
We also know that this explanation isn’t a “legitimate reason” for the prejudice this question asked about. It’s a rationalization. Prejudice isn’t built on “reasons,” but it does like to find rationalizations to give the hate, mistreatment, etc., a guise of respectability, a guise of “What can I do? It’s just the way things are!”
(We don’t have to go farther, though if we do, others have pointed out that the evidence we have goes against the claim itself and the notion that this was where the prejudice began.)
To be clear, I know you don’t personally hold those feelings. But it just seems to me that by saying you weren’t there so you can’t know, by saying it’s “their choice” to believe this or not, you are defending the prejudice that leans on this explanation, even if inadvertently, by treating this explanation as a valid “reason” to have those feelings.