“Resident Evil” is a zombie. Once one of the most respected and beloved video-game franchises, it’s now a shell of its former self. It lumbers along brainlessly, even though nobody can explain what’s motivating it. And no one can stop it from reproducing: This year alone we’ve seen three “RE” games as well as the latest movie spinoff.

The zombies in “Resident Evil 6” have evolved since they were first unleashed 16 years ago. Sadly, the series itself has devolved since its 2005 peak, “Resident Evil 4,” forgoing its roots in horror in favor of a more bombastic action-movie vibe. It trades in spine-tingling frights for ear-splitting explosions. It feels more like “Call of Duty,” except the zombie-killing segments in recent “CoD” releases are so much more fun.

The “Resident Evil” mythology has gotten hopelessly convoluted over the years, and “RE6” doesn’t improve matters, splintering the narrative into four campaigns. The stories begin with different characters in different places — the United States, China and a fictional Eastern European country — but they eventually all have to contend with the C-virus, a biological weapon that has made the zombies tougher than ever.

Some of them, called the J’avo, are relatively intelligent and can even use firearms. And that’s where the latest installment starts to collapse. Pistol-packing monsters just aren’t scary — the nightmares happen when they get close enough to knock you down and start gnawing on your entrails. So by turning most encounters into gunfights, Capcom has turned “Resident Evil 6” into an ordinary shooter, a third-rate product in a glutted genre.

With 2009’s “Resident Evil 5,” Capcom made the misbegotten decision to rebuild the series around two-player, cooperative action. “RE6” doubles down on that mistake, and it’s only fitfully amusing to play side-by-side. If you play solo, you’re bound to get annoyed with your computer-controlled partner, who takes a long time to help you open doors when you’re trying to escape from a roomful of ravenous freaks.

The weirdness of forced co-op play comes to the fore during boss battles. Several times I wound up slinging ammo at the game’s biggest mutants, with no idea whether I was making a dent. Then, suddenly, the battle would end, thanks to some unseen maneuver pulled off by a virtual teammate.

In a few cases, boss battles are resolved with quick time events, in which you have to match on-screen sequences of button pushes. Sometimes, though, the only way to survive is to just keep pounding buttons and hoping for a break. Even then, “RE6” returns all too frequently to the gimmick of having a defeated boss return in the next act, more powerful than ever.

There are other flamboyant sequences that try to break up the core human-versus-undead conflict. You may find yourself running from a behemoth, or steering a snowmobile, or sniping at motorcycle-riding J’avo. To Capcom’s credit, “RE6” keeps trying new things, but few of them are well executed.

The original “Resident Evil” launched a boom in horror video games, inspiring gems like “Parasite Eve,” “Silent Hill” and “Fatal Frame.” But “Resident Evil 6” kills the suspense, dragging players from one chaotic set piece to the next and shows only a few glimmers of what made the series so popular.