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

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.

the lowest among the social higher rank were the working class. this class remained detached to the political progression   of the country and was hostile to the other two classes