The horror sequel “28 Weeks Later” has the most arresting, gruesome and unnerving opening 11 minutes in movie-going memory.

The follow-up to the “Britain Wiped Out by Rage-aholic Zombies” thriller “28 Days Later” briefly and economically introduces us to a small clutch of survivors, walled-up, “Night of the Living Dead”-style, in a remote farmhouse. Then bloody-eyed Brits pour in and slaughter everybody who isn’t able to beat them off with a crow bar or outboard motor.

“Weeks” is a frenetic killing machine – illogical, telling in its grasp of human nature and utterly incapable of embracing the humanity it wants to show us. It’s reduced the best horror franchise of the new millennium from a smart European dystopia to another Hollywood killing machine, brutally efficient and heartless.

Director Juan Carlos Fresnadillo, who did the stylishly surreal “Intacto,” beautifully sets us up for a grim tale of survivor’s guilt as “Full Monty’s” Robert Carlyle must live with the knowledge that when the chips were down, he cut and ran, leaving his wife (Catherine McCormack) to a grisly death. Since their kids were away from the UK when the Rage Virus broke out, Dad may even have to explain his cowardice to them when the “re-population” begins “28 Weeks Later.”

Maybe Dad thinks, “Well, at least one parent will be there to raise them.”

The big, all-encompassing message of “28 Weeks” is that human nature – compassion, empathy, a mother’s urge to save a child – is fatal. If there’s a modern-day metaphor here for the West and the Middle East, here’s a movie that comes out on the side of genocide. Sympathy is weakness. Empathy – for children, innocent civilians, parents and your own offspring – will get you killed.

Because the kids, the poster-pretty Mackintosh Muggleton (his real name?) and Imogen Poots (ditto?), are trouble from the moment they’re brought back into “The Green Zone,” a resettlement camp in London guarded by U.S. troops. The kids show up, get a version of how Mom died from Dad and promptly sneak out of the Green Zone to collect stuff from their old house – and find Mom still living.

Dad’s got some explaining to do. But what about Mom? She carries the virus, but has survived the disease. Maybe Army Doc Scarlett (Rose Byrne) can find a cure.

But before that can happen, guilty Dad and forgiving Mom are swapping spit, Dad loses it and the Green Zone turns Red in a blur of blood and bullets. The Army will have to “cleanse” the place. Thousands will die. The rest will become zombies. The doc and an Army sniper (Jeremy Renner) who grows a conscience and starts shooting his own men struggle to rescue the kids who caused all this mayhem.

Forget the movie’s jaded attitude about the coarse and callous U.S. Army. What soldier starts shooting his comrades on a whim?

The effects in this are simple – a set piece involving an unarmed helicopter being used against the living dead, the vivid napalming of central London. The best effect is the simplest of all – an empty city and countryside. The quiet skies are clear of planes, birds and pollution. Streets with abandoned cars and pizza delivery scooters, and only the occasional skeleton to spoil the view.

The sequel takes the story even closer to the bleak heart of the film’s obvious inspiration – “The Omega Man.” But the plot is absurd in the extreme, as chemical weapons are survived by holding a shirtsleeve over one’s face, characters take every opportunity to go into tunnels and dark places and raging, demented Dad is still clever enough to track his kids hither and yon with a notion of ripping their flesh and making them just like him.

We watch. We endure. Because perhaps, we hope, he’ll stop spitting blood and growling long enough to tell the brats the line we long to hear, from the 30-minute mark to the finale:

“This is all your fault.”