Tl;dr - while slavery is certainly too harsh of a word the conditions of the Southern peasantry of the time were far from ideal.

Historically, the lower agrarian classes of Sicily (and Southern Italy) did not own the land they cultivated; much of it was divided into large, unproductive latifundia with most of them being in the hands of a small number of mostly noble, mainly urban and often absentee landlords. Due to the aforementioned state of things the institution of mezzadria (tenanted farming), while commonplace in the Central and Northern parts of the Peninsula, was not widespread in the rural South whose peasants led fairly miserable lives (even by contemporary standards) and were for the longest time denied equal access to the same land they were tending to.

It could be argued that the post-Unification governments - despite all intentions, promises and reports - may have exacerbated things further by extending the Piedmontese system of taxation (which carried a far greater fiscal burden compared to the relatively light one formerly employed within the Kingdom of the Two Sicilies) and a number of new taxes to the newly-acquired South; these taxes, most notably the one on flour, targeted essential items and foodstuffs. You can very well imagine how such a system - tailored on the more prosperous and industrialised North - could have damaged the already fragile Southern economy and its society as a whole.

The industrial policy of the new State tended to favour the North, under the flawed assumption that it would've acted as a catalyst of development and somehow help pull the South out of its misery. All it achieved was diverging even more investments away from it.

Moreover, the disturbances (late 1860s-1870) caused by these taxes not only had to be brutally quashed by the Army but left the countryside at the mercy of innumerable acts of brigandage, further worsening the poorer strata's already low standards of living. The central governments were pushed into seeking an accommodation with the landed gentry (and by extension the many people, such as field guardians, it employed) with the goal of putting an end to the commotions and keeping the peasants under check. A situation that lasted well beyond WW1; keep in mind that the first nuclei of the infamous fasci di combattimento were formed by war veterans hired by landlords with the task of intimidating and/or beating the living pulp out of all those peasants who dared protest or fight in favour of a more equal redistribution of land... something that would only materialise decades later when the long-awaited riforma agraria was passed and enforced, not without some opposition, in 1950-51.