This hobby is weird like that. People don’t tend to insist on only watching one brand of movies or eating one brand of food, or one board game, or listen to only one band, but D&D and Pathfinder in particular have fan bases which a large portion of won’t ever even consider another game. That doesn’t seem so common to fans of other RPGs (though they exist of course).
There were many years I would only play D&D. Luckily I grew out of that and discovered heir were so many wonderful gaming experiences I was missing out on.
Is it really that much different from sports fans that only support one particular sport and not another?
I have seen worse stories about sports fans then RPG fans, even the famous "edition wars" resulted in hurt feelings but no deaths.