You are enclosing the field
AND value in quotes, you should only enclose the field; it should be 'Field'=NULL, not 'Field=NULL'. Quotes aren't really needed at all, unless there are spaces in the field, but that's not a good naming practice to begin with.
Code:
UPDATE `rank_system` SET `kills=NULL`,`deaths=NULL`;");
Code:
UPDATE `rank_system` SET `kills'=NULL,`deaths'=NULL;");
__________________