I'm not aware if there's some account for the direct feelings of the at the time Romans on this very event of crucifixion; at least I haven't tracked one. Possibly, as this Servile war was the 3rd [73−71 BCE], after about 80 years of slave rebellions, thoughts on slaves' status should have arisen among Roman citizens, either for good or bad; just a rational conclusion.

Cicero shortly after, in 70 BCE ca, is giving an aspect of the capital punishment by crucifixion in his famous speeches 'In Verrem'. [*Cicero uses the term 'crux' and derivatives. However some scepticism has been expressed for if it's crucifixion as we are meaning it].

There he accused Gaius Verres, praetor - governor of Sicily, for misgovernment. Inter alia [Cic. Ver. 2.5.158ff] Cicero is mentioning a case of a certain Publius Gavius, a Roman citizen who was crucified by Verres under the charge of spying for Spartacus' slaves' favor; though Gavius was a Roman citizen [Cic. Ver. 2.5.161 & .164]. It's clear that according to Cicero crucifixion isn't a punishment that should be applied to citizens, while it's described as quite cruel.

However, he doesn't seem to share the same opinion regarding slaves. Earlier in the same speech [Cic. Ver. 2.5.7], Cicero is mentioning an older law by M. Aquillius [probably the consul of 100 BCE ca] that no slave should carry weapons. And by the way he's narrating a known incident around a following governor of Sicily, Lucius Domitius. At an instance Domitius asked amazed how a certain huge boar was killed, and it was answered by a shepherd with a spear [=venabulum]. The shepherd - slave was sent instantly at the cross. Cicero said this might be considered harsh [=durum], but he explicitly avoids to judge Domitius' decision, saying just that the governor chose cruelty over overlooking.

Slaves were probably considered worthy of every maltreatment.
Of the first who wrote on Spartacus' war was Roman Sallust of the 1st c BCE [SallHist 3.64-66], but only short excerpts have survived from his account, according to which a distinction is implied between revolted prudent slaves with noble minds/souls & revolted savage ones. Sympathy to a degree? So let's see how the events of the Spartacus war were seen by three major sources; Florus, Plutarch and Appian, who is the only mentioning the crucifixion [and maybe it isn't 100% clear that Appian with the use of the verb 'κρεμάννυμι' is meaning 'crucifying' or just 'hunging'].

Roman Florus of the 2nd c. CE [Ep. 2.8] is thinking of 'armed slaves' as a disgrace [=dedecus]. He seems sympathizing them somehow as a '2nd class human beings' who could, nevertheless, be treated in any way but who could also be freed. However he states that he doesn't know how to call this war, while he's mentioning the slaves as the lowest [=infimae] and the gladiators as the worst [=pessumae]. He then calls them 'rabid monsters' [=rabidis beluis], while he declares ashamed to call them 'enemies' [=pudet dicere — hostes]. It's obvious that he's totally underrating them and there's low-appreciation.

Greek Plutarch of the 2nd c. CE [Plut. Crass. 8-11] is either calling it as 'gladiators' revolt' [=μονομάχων ἐπανάστασις] or 'slaves' war' [=δουλικῷ πολέμῳ]. He's also mentioning 'injustice' against the revolted as the cause of the rebellion; while he's writing that many herdsmen & shepherds joined Spartacus. It's unclear if these were all slaves or free-men. In any case, is this implying a more general acceptance of the rebellion? I can't know. Regarding Romans, at some point Plutarch describes how the disgrace of this revolt, that the senate felt, gave its place to fear of losing both of Roman consuls. But further he's giving a hint that the opponents were considered lesser and inferior, at least regarding fame. According to him, while Pompey celebrated a triumph for his victory over Sertorius of Spain, Crassus didn't even ask for a lesser celebration for his own victory over Spartacus, as it would be for a servile war, thus unworthy. Again a Roman aspect of low-appreciation for the slaves. However, both Pompey & Crassus, became Roman consuls the following year.

Roman Appian [of Greek origin] of the 2nd c. CE again, seems to be the only one who is mentioning this 'crucifixion' of the 6.000, and it's maybe the most detailed of the surviving accounts [App. BC 1.116-121]. It's noteworthy that he's mentioning free men joining Spartacus along with slaves; more clear compared to Plutarch. However he's also adding some events that could indicate fear and desire for vengeance by the Romans. He's narrating for instance that when Crixus [Spartacus' companion] was defeated and killed [72 BCE], Spartacus sacrificed 300 Roman prisoners to honor him. This probably caused some feelings of fear. Continuing Appian says at some point that "The war was already 3-year old and terrible to them [=Romans], even if in the beginning was derided and despised as [a war of] gladiators'". So when the Roman military leadership was decided to change, all hesitated but Crassus. Then it's described how Roman army's moral had increased in a battle cause of a previous victory-massacre over slaves, that is called 'punishment' [=κόλασιν]. A clear implication of a previous fear of Romans and maybe of slaves' status, as they were punished, not just defeated. Further, Appian describes when Spartacus himself crucified a Roman prisoner in the middle of a battlefield so show to his men what will suffer if they failed. It seems that there was a certainty for a revolted slave's fate.

So at the time, Romans seem to think of slaves as lesser human beings, on whom could be applied the worst punishment-maltreatment. They also seemed unworthy opponents regarding glory. They didn't deserve to even hold arms. While Spartacus and his slaves had surely terrified them. According to these I'm not sure if crucifixion of revolted slaves, could be considered exactly an atrocity as we think of it nowadays; taking also in account that it's possible that crucifixion was a known capital punishment for slaves. Florus [Ep. 2.7] for example, describing the 1st Servile war in Sicily [135–132 BCE], is mentioning around the fate of the surviving slaves: 'crucibus'.

However, there's no total certainty on this. Cause also a sympathy can be traced occasionally, based eg. on some claimed injustice or a possible participation by free men, too, or even just by a human aspect.