Robert Byrd left the KKK in 1952, and in 2005 gave a public apology for his involvement with the organization.

David Duke, while having left the KKK, is still a self proclaimed white nationalist, and has given no public apology for his Klan involvement in the way that Byrd has, which was to denounce white supremacy altogether.

Hillary Clinton's comments about Byrd being her mentor came at a time after Byrd had converted from a bigoted ideology, and atoned for it. Why shouldn't such a person be a mentor? Byrd died an honorable, great American and great public servant, and this was partly because he'd denounced the KKK and white supremacy.