Well that was depressing. And I mean that in the best possible way. It was a really good story, well told, the pertinent details revealed at a good pace so that it was neither info-dumpy nor confusing. With the opening scenario I suspected lycanthropy (chaining up the werewolf so he can't hurt anybody), but the actual reveal was much more interesting. Lots of interesting food-for-thought here from deteriorating loved ones and deciding if/when to let them go, the effectiveness of weapons to inspire terror vs. their effectiveness to kill or subvert the enemy, the tendency of biological weapons to be very hard to corral once released (and this one wasn't even contagious, but still impossible to consider it completely eradicated).
Dave did a really excellent job with the narration too, lots of good emotional expression there.
I didn't find the ending where love conquers contrived at all. The reason is that the one who was swayed by love was not Patient Zero. It was a merged personality of Patient Zero and Sergio. He was designed so that his hatred would come through while retaining the memories of the infected, but with the memories came the emotions and personality of the person as well. Sergio held very strong beliefs, and his love for the narrator was very strong, and both those things came through in the final result of the transformation. This Patient Zero is still full of consuming hatred, but because his mix of past experiences is different than other incarnations, his hatred of his enemy in the war has become a hatred for the futility and random violence of his part in the war effort.
In the end, the result of this small encounter makes no earth-shattering difference. Sergio still dies. Our narrator still loses his beloved. Patient Zero's mission is no less futile. Which is all rather depressing, even though it fits the story.
What I really liked about the ending, though, is that Patient Zero redeems himself to some degree (at least in my eyes). Before the ending, he is apparently a hate-filled killing machine. But this repeated incarnation process supports a good argument of nature vs. nurture. I got the impression that at the end of the transformation he will essentially be the same physical person he was, but with some physical enhancements and with other memories. So physically, his nature is the same, but robbing a new set of overlaid memories from the host allows a change in nurture. Most of the time, the Patient Zero incarnations have done violent and hateful things when they came to be. But the fact that THIS one didn't suggests that the violent and hateful nature that shows through in most of his incarnations is not an inevitable part of who he is--if he had had a different upbringing he might have become a gentler and less hateful person.
I don't know if this was intended at all, but for me the character arc of this story is centered around Patient Zero, not around the protagonist as one might guess. And I thought it was done very effectively. Even that is tinged with some hopelessness because other Patient Zero's will not remember this incarnation, but it reveals his potential for gentleness to the reader and to the narrator.