Crucifixion was called the servile supplicium
(punishment for slaves) in Roman times. Crucifixion was not, however, solely the prerogative of emperors, prefects, legates, and the like; apparently every male head of a household (pater familias
) technically possessed the power of life and death (ius vitae necisque
) over the members of his house, including wife, children, and slaves.
(I am given to understand that this absolute power was gradually eroded over time, in stages, from Hadrian to Antoninus Pius to Valentinian I; if anyone can shed light on that process, it would be appreciated.)
Martin Hengel writes on pages 57-59 of Crucifixion in the Ancient World
The conflict between the orders of a master and the commands of the state, both of which threatened the slave with crucifixion, or between the goodness of a master and the limitations of class, of which crucifixion was a symbol, became a favourite theme of rhetorical declamation.
Slaves thus had relatively little protection against the whim of their masters and therefore against unjust imposition of the servile supplicium. The dialogue between a Roman matron and her husband, given by Juvenal (6.2i9ff.), says more here than many examples:
'"Crucify that slave", says the wife. "But what crime worthy of death has he committed?", asks the husband. "Where are the witnesses? Who informed against him? Give him a hearing at least. No delay can be too long when a man's life is at stake." "What a fool you are! Do you call a slave a man? Do you say he has done no wrong? This is my will and my command: take it as authority for the deed."'
('Pone crucem servo!' - 'Meruit quo crimine servus
supplicium? quis testis adest? quis detulit? audi;
nulla umquam de morte hominis cunctatio longa est.'
'O demens, ita servus homo est? nil fecerit esto;
Hoc volo, sic iubeo, sit pro ratione voluntas!')
In his defence of A. Cluentius, Cicero accuses the mother of the accused of having had a slave crucified and at the same time of having had his tongue cut out, so that he could not give evidence (Pro Cluentio 187). ....
Of course there was also criticism of excesses of this kind. For Horace, a master who has his slave crucified because he surreptitiously tasted some fish soup while bringing it in, 'is quite mad by any reasonable standard'. This attitude was matched by Augustus' tendency to curb the whims of slave-owners in favour of the authority of the state. Seneca even went so far as to remark with some degree of satisfaction 'that the cruelty of private slave owners was avenged even by the hands of slaves, who stood under the certain threat of crucifixion' (sub certo crucis periculo, De dementia 1.26.1).
I suppose it was all down to what kind of master a slave had. But masters crucifying their slaves seems to have been common enough to elicit the above kinds of responses and complaints, at any rate.