Despite Zeref's strong dislike of Kefka, it would take him being targeted specifically for him to get involved, due to him just flat-out not liking fighting. If Tartaros got involved, he'd have no choice but to join in as well.

Their guild hall is located in Everglade, which does have a guardian. It's not an easy place to attack since it's set up in the city catacombs, but if Kefka's cultists did do it, it would finally be the last straw for Zeref.