What evidence seems to be the most valid (true or reasonable) and which evidence seems to be the least valid in this case?

I think that the knife being so common that anyone can get one disproved that it was a "one of a kind" weapon. Many of the jurors had stereotypes about kids who grow up in slums—and who belong to certain minority groups. This was often handled as evidence that the boy was guilty. The boy's ethnicity should have had nothing to do with the murder.