Exactly what happened 65 million years ago is debated since with a lot of the evidence it isn’t completely clear exactly when things happened. One thing you may want to factor into your doomsday scenario though is fire. There’s a layer of soot all around the earth which tends to indicate that the entire world burned after the Chicxulub impact. Exactly how and why is also debated, but it’s easy to imagine that the impact throws up all kinds of hot stuff and it literally rains fire as this stuff floats around the earth and comes back down.
Cities are going to burn. Food supplies are going to burn. People are going to burn. It’s basically going to be hell on earth. When 99 percent of the world’s food supply disappears overnight, there’s going to be all kinds of violence as the survivors scramble for the remaining 1 percent. So, it’s hell on earth with the survivors shooting at each other while the world burns around them.
This isn’t quite as instant as maybe you are looking for, but it’s not a slow dwindling death. We’re talking about a major population loss in a very short amount of time. There will be small pockets of survivors all over the place, but maybe that’s what you want anyway.
Another thing to consider is that the shock wave spreads out as it travels around the earth and comes back to a point on the opposite side of the earth from the impact. You aren’t just going to get major earthquakes at the site opposite the impact, you’re going to get a crust breaking major event that’s probably going to spark off all sorts of volcanic activity.