Matthew Morris -> The simplified version of what Mav is saying (I get the sense that you need it simplified for you) is that you are comparing individuals to a movement.
Some individuals who claim association with BLM have committed horrid acts, including murder. The average BLM associated person condemns the majority of these horrid acts (as Mav has pointed out, he condemns all the acts except for the property destruction). The organization as a whole does not publicly (or privately, but you wouldn’t know that) call for violence as one of its founding principles.
The KKK as an organization (and other white supremacist groups) does explicitly call for violence against non-whites as a founding principle. Individual members who commit these acts aren’t defying their organization, they are directly supporting it. The KKK even applauded the murder of Heather Heyer after it happened, basically because she was a “race traitor.”
That is the difference that you fail to understand (possibly because a black man tried to explain it to you instead of a white man).