Why would killing every slave that joined Spartacus send a message to keep
other slaves from revolting?

Answer:
Spartacus ordered his army to turn back north, but as they made their way, Crassus and his legions met them. .Crassus attacked the slaves and demolished them. It is believed that Spartacus died in this battle. The Romans later crucified some 'six thousand slaves on the main road to Rome.