Robert E. Lee fought on the confederate side which was the southerners. He owned slaves and was for slavery. So white supremacists see him as an symbol for slavery, and since he was for slavery like how most southerners wanted he fought for that. Now that slavery is mostly abolished everywhere (there is still slavery going on) the white supremacists don't like that and are against it. They think they are more superior than any other race. With the Virginia protests with removing the statue the group of the supremacists said "you will not remove us" or something close to that I forgot. This is off what I remember because I just did an assignment on this. Also they think he was a good general because he fought for white supremacy, so yes he was a supremacist. That is why so many racists are protecting his statue. There is an article that I can send you in your pm if you would like.
Hopefully this somewhat helped :)
