[…] a stupid-looking, wide-mouthed creature with bewildered hair and face […] in any case, the condemned man looked so like a submissive dog that one might have thought he could be left to run free on the surrounding hills and would only need to be whistled for when the execution was due to begin. (1)
Yes, the condemned man is basically depicted as a dumb animal, and that's sort of the role he plays throughout the story. For much of it, he's kept in chains, and choked back whenever he moves too close to the apparatus. One of the most notable things he does is vomit, all over the apparatus and himself.
He also never talks – he doesn't understand the French of the officer and the explorer, and so just sits by, watching them with a kind of mute fascination. The one sentence he does say, although we only hear of it through the officer, is "Put that whip away or I'll eat you alive." It doesn't help the case for your humanity much if you only speak to threaten to eat people.
That little fact makes it clear the condemned man is actually a pretty brutal guy, in addition to being somewhat animalistic. Despite the horrible fate he's supposed to have, he's just not very sympathetic (the explorer doesn't feel any sympathy for him, really). He's also eager to watch the officer suffer through the pain of the apparatus.
The condemned man does make us wonder how he got to be this way. Was he just a barely human criminal before he got to the penal colony? Or has the harsh discipline and brutal treatment of the penal colony made him this way? Perhaps it's a system that demands such total submission of the inmates that it turns them into animals. And it certainly looks as if the condemned man is conditioned for submission – when the explorer tells him to go away, he gets right on his knees as if to beg.