Montgomery is Moreau's assistant on the island, and we know little else about the man, his past, or his motivations. We understand he left London society after he "made a young ass of [himself]" (2.21) and "lost [his] head for ten minutes on a foggy night" (4.12). What exactly he did is punctuated with a big-old question mark. We are never told how he came to know Moreau or why he decided to work for the doctor.
Maybe more importantly, we are left in the dark as to why he stays with Moreau. Moreau's work clearly agitates Montgomery emotionally; in chapter eight, the man winces every time the puma cries. He could have escaped anytime he went to the mainland for supplies, so why keep coming back?
For that matter, why did Wells choose to have Montgomery be such an ambiguous character? Why not just tell us about him and his past? It adds suspense to the novel, sure. After all, there are many dangerous times when Prendick must rely on Montgomery, such as before he discovers Moreau's secret and after Moreau dies. Having such a mysterious character certainly makes it iffy as to whether or not Prendick is really safe with Montgomery. But then Montgomery dies, and we still don't know anything about him. Why?
Wow, there's a lot of whys up there, but we've got one more. Why do you think Montgomery stays with Moreau? Was what he did that awful or does something else keep him there?
One possibility is that Montgomery stays because he truly does care for the Beast Folk. Prendick believes he has "been with them so long that he had come to regard them as almost normal human beings" (15.10). And that seems pretty accurate.
Between Moreau, who wants nothing to do with them, and Prendick, who is alternatively terrified and disgusted with them, Montgomery interacts with them regularly and with the most ease (14.47). He even tries to party with them, with what we'd call mixed results (see below).
Maybe he sees Moreau's experiments as necessary—though in a different way than the doctor. Moreau considers his experiments important because they allow him unlock to the mysteries of the cosmos. For Montgomery, the experiments are a necessary evil, granting him a chance to replace the social life he longs for with this mockup society. Although the pain he causes the animal is excruciating, he knows it will provide another member to the community. And with Moreau's growing skill, one closer to being a human companion.
As we said though, this is just one possibility. Can you think of any others?
We don't mean that Moreau shaped Montgomery from an animal for the express purpose of partying (though that would have been a cool plot twist). No, Montgomery is a beast man in the sense that he is wholly integrated into their society. This idea is especially evident when Montgomery is drinking.
With bottle in hand, Montgomery deadens himself to the pain of others as well as his own. For example, while listening to the puma's cries, Montgomery "help[s] himself to whiskey and water with great deliberation" to ease his conscience (8.9). He also brutalizes poor M'ling, puts Prendick's life in danger, and, eventually, is killed in a brawl with the Beast Folk. All of this while drunk as a skunk. It's as if the civilized man drains out of him as the brandy drains in.
Montgomery's last words are, "[…] the last of this silly universe. What a mess —" (19.39). It's entirely possible that Montgomery already knew or suspected what Prendick only discovered at the very end of his journey (check out the "What's Up With the Ending?" section for more). Realizing the inescapable beast in himself and those around him, Montgomery succumbs to the same depression Prendick does, only without hope for escape.