Italians enslaved other Italians (but the victims were never Roman citizens so it didn't count)

Romans could be sold into slavery to pay off their debts.

The Romans were reletively free of out-group hostility - they felt the barbarians outside the empire were savages, but they tended to absorb local power structures and religions, granting the local nobles (if they cooperated) Roman citizenship, (which was more exclusive than, say, American citizenship,) and while there was some generic snobbery there does not appear to be any belief that non-Romans were inherently inferior. Once they joined the empire, they gained all the rights and privileges of your average Roman (including protection from those barbarian savages over the hill.)