What does James mean when he says that Judging a Brother Equals Judging the Law?

>James 4:11 Speak not evil one of another, brethren. He that speaketh evil of his brother, and judgeth his brother, speaketh evil of the law, and judgeth the law: but if thou judge the law, thou art not a doer of the law, but a judge. How is judging a brother equal to judging the Law? What does this mean? Then again if 'A' judges 'B' as wrong (this I understand as based on the Law) and speaks evil of 'B', how is 'A' speaking evil of Law? Is there a logic behind this reasoning?