What I mean it, what if KILL would ignore WSREP_TO_ISOLATION_BEGIN
failure and will just proceed killing? Perhaps if
WSREP_TO_ISOLATION_BEGIN fails it means that there can be no bf aborts
anyway? Could you try to find it out?
After TOI error that node will not serve user anymore, you either get
ERROR 40001: WSREP replication failed. Check your wsrep connection state and retry the query.
or
ERROR 08S01: WSREP has not yet prepared node for application use
User KILL will return the first one. bf_kill can't happen as you can't issue commands to cause it.