Its been used by people for hundreds of years and spoken in indigenous tongues. Its usually translated though as white "devil" but really its just white "evil thing" I think Arabian muslims refered to whites as white "devils" as well.
As for whether people think that whites are devils, their history around the world should answer that question for you. From the purposeful genocide of the native Americans, working to death of Asians in North America, horrific treatment of African slaves it just goes on and on. Every time they interacted with a different culture they ended up making that people worse off.
People can take this history and the system of white supremacy that is currently in place together and say that whites are just inherently evil, and because of their apparent lack of empathy and willingness to commit horrible atrocities to fellow human beings for profit that they are in fact evil or sponsored by the devil itself(in fact I believe the founder of Satanism is a white man as well as the majority of its proponents)
The phrase "white devil" is not used as a playful insult. It is usually stated with some real hate behind it which again comes from the history of the word which is tied to the atrocities that said people have committed in the past and continue to commit today.
Personally I don't think that most people are inherently evil, especially not an entire race of people. I've encountered quite a few white people that understand their history and how people perceive them and are moral enough to fight against white supremacy.
you went in on this... hard to find white people like that i honestly think they are none 
