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"Defection" has been covered as "voluntary turning" (namely what Orwell, the lookamancer from FAQ did by turning to Haffaton). Leaving one's side in order to found a new one would differ only insofar as it involves creating a side at the same time as turning to it.

And personally, I initially read Cat's words during dinner from Forecastle's perspective and still didn't think it was particularly hurtful. Maybe I'm making inaccurate assumptions about his personality or something?