Thievul are far too large to steal like Nickit do, by nabbing an item when the owner is not looking and scampering away before they're noticed. But this is of little consequence, for it takes a very brave person – or a very strong trainer – to meet a Thievul on the road and refuse to give it whatever thing of theirs it desires.
Unlike Nickit, Thievul need not steal food, for the surplus gathered in their tails as Nickit can last a very long time; it is not unheard of for an elderly Thievul to pass away with its tail still containing edible nuts it had stolen before evolution. Should that supply prove insufficient, they are large enough to supplement their diet by hunting; instead, they steal for fun.
Although it is true that Thievul steal exclusively from Galar's aristocracy, it is unlikely, despite their common portrayal in Galarian culture, that they do so as avengers on behalf of the common villager. (A term which, like villain, is itself commonly derived from a dialectical pronunciation of Thievul's vocalizations, although other possibilities, such as the related Vulpix, can not be ruled out.) Nor do they do so as revenge for being hunted as Nickit; icons of chivalry who eschewed hunting were no more likely than their peers to avoid the Thievul.
Wild Thievul do not perceive the structures of human society, but they desire the jewels and capes restricted by fashion, wealth, and sumptuary laws to the elite. It is true that they carefully select their targets, but this is to restrict the challenge, not to increase it; those with sufficient bodyguards nearby may fight back. Once successful, they leave their treasures in nests as well-hidden as a human's secret base – for themselves, their children, and, centuries later, archaeologists to admire.