The narrator tells the story from the explorer's point of view, which makes it relatively easy to identify with him. He's sort of in the same situation as the reader: he's the outsider in this weird penal colony with its strange judicial system and horrifying torture device. Furthermore, his "Western values" – humaneness, a fair trial, etc. – are supposedly our own, and his reactions to the penal colony are probably very much like ours. Although he doesn't do very much besides watch, it is fair to say that a lot happens to him (the whole story does), and what happens to him in the nightmarish world he's stepped into moves him to escape it as quickly as he can.
The officer's a less obvious candidate for the role of protagonist than the explorer, since the story isn't from his perspective and he seems so strange to us. We also doubt his sanity. If you look at him as a brainwashed victim or a narrow-minded zealot, you can't put him in this role very easily. But if you find something admirable about him – maybe in the strength of his conviction – you can definitely place him as the protagonist. Don't you feel bad that his whole world goes down the tubes? And isn't there something kind of heroic about his suicide? Maybe?