Why did Spartacus desert the Roman Army?

Spartacus, joined the Roman army at 63 BC. Spartacus was poor so he joined the Roman Army hoping he will able to fight for his freedom but, Spartacus was asked to march agaist he own people, meaning he will have to kill people in his hometown, Thrace. But Spartacus does not agree to fight/kill his own people like saying "No I'm no doing this" then he deserted.