Well...he would be killed on place or his bounty would still be out there...and his excommunicadostatus would still be preserved. I mean they would probably also want him dead. And the Elder would be probably replaced by someone next in lane to watch over the High Table. Or maybe some1 from the High Table would be elected as new Elder. We don´t know yet enough about the Table.
He didn't locate the Elder, the Elder found him. We don't know nearly enough about the Elder or how the High Table is run to guess what would happen if John had killed him. Probably the same as if the guy died of natural causes.