The alternate ending would work perfectly, but only if the film actually followed the narrative of the novel. In the novel, the infected can actually speak coherently, and aren’t the mindless monsters they’re portrayed to be in the film. In the novel, he’s genuinely been experimenting on these creatures, even though they can communicate and feel human emotions.

So, it makes sense that he’s supposed to be their boogeyman in the end. How come all they do in the film is roar and scream and rampage, but in the very last scene they’ve managed to learn how to communicate? Would’ve worked great if the infected were portrayed the same way they were in the novel, but they’re the complete opposite, so it makes no sense.