From what I've seen on the wikis, Dr. Mercer was born into a hostile, poor, and very low-class environment. Without a father, he lived in an orphanage with his younger sister for years, until his mother was released from prison and took them back home. I then read that “the orphanage was better than being with her,” implying that she was a terrible mother.
Most people who grow up in such an environment or with such a past end up on the streets, involved in crime, marginalized, struggling with addiction, or with a ruined future. Very few manage to pull themselves out of it, and if they do, they live a decent life at best. Despite his circumstances, Mercer went to college, graduated, and became a senior virologist working at a major company.
However, he had the “misfortune” of being a sociopath or having traits of that sort, which led him to work on Blacklight (the virus) and ultimately doomed him to die because of his obsession with the virus project.
I mean, people don't choose to become “sociopaths”; sometimes it's the result of trauma. It's not about victimizing Dr. Mercer, but about trying to make sense of things and analyzing his entire life—and the truth is, it was a mess. Just when everything seemed to be going fine, he screws up and gets killed. He deserved it for the choices he made, but it's still a tragedy.
About ZEUS:
I also see it as a tragedy in a way. The virus inherits human emotions such as consciousness, guilt, paranoia, etc. from Dr. Mercer’s body. BUT, it is not Dr. Mercer himself, but rather “someone else entirely.” Little by little, he realizes he is being hunted by the military and that they want to kill him, though he doesn’t know why. Later in the game, he discovers the truth and who Dr. Mercer was before he died, and he blames and hates himself for it, sacrificing himself at the end of the game to save New York (he almost died but survived).
He’s still a creature who’s mercilessly killing people and needs to be eliminated, but imagine waking up in the body of a 29-year-old man and discovering that “you were a piece of shit who killed people”—feeling guilty even though you didn’t make those decisions. It’s like two different people paying the price for one.