but while most anti-McKeller fans direct their resentment and frustration at the show itself and the writers (rather than flaming McKeller fic writers or the like), some pro-McKeller fans, rather than just squeeing, also direct a portion of negativity toward the anti-McKeller fans, accusing them of bringing the fandom down or all being rabid McSheppers who can't appreciate het relationships.

I haven't seen instances of the two examples of fan-on-fan disagreement you offer at the end of the paragraph I've quoted above, but I have no problem with McKeller fans taking issue with some of what anti-McKeller folks have said. Keller's been called a bitch, a whore, a slut, and a flake - it's not the show writers who are writing her that way; it's fans who are interpreting her that way, or worse, expressing their dislike of the character in profoundly gendered, derogatory, sexual ways. Not all fans are doing it, nor are all fans' disagreements with Keller based on those terms, but it's absolutely undeniable that a lot of trash has been talked about Keller in base, offensive ways.

I'd also argue there is no "anti-Keller" side to face off against the "pro-Keller" side, nor an "anti-McKeller" to face off against "pro-McKeller" - those big terms hide a multitude of disagreements, interpretations, and ideas. I would no more paint everyone who dislikes McKeller with the same brush as I'd paint the people calling Keller a slut than I'd paint everyone who likes Keller with the brush that suggests they're taking things too personally.