Ya see, it's kinda like the definition of a weed, viz: a plant growing where it's not wanted. So,
Bloodthirsty Sunni thugs fighting bloodthirsty Shia in Yemen, Sunni = good guy (well, for restricted values of good).
Bloodthirsty Sunni thugs fighting bloodthirsty Shia in Iraq, Shia = good guy (well, for restricted values of good)
Bloodthirsty Sunni thugs fighting bloodthirsty Shia in Syria, well, it's complicated.