It's not so bad when you have time to stop. The carnage is always worst during mating season - the deer are too preoccupied to know, or care that there are huge metal things rolling along at fifty miles per hour and spelling doom and destruction for their uneventful, herbivorous lives.
Deer evolved in a world where the greatest thing to fear was a pack of wolves. The wolves, for the most part, are history, leaving the deer free to multiply and overpopulate and dash across country roads. Their instincts just don't prepare them to deal with an oncoming automobile. They can't decide what to do, and their brains aren't equipped with the tools for an override, so they freeze on the pavement, unblinkingly staring down death in the form of a Ford Econoline rolling along at fifty miles an hour with its high beams glaring ahead.
A big deer being hit head-on by a small car at high speeds is usually a death sentence for all parties involved. Larger automobiles provide better protection for their passengers, though not the deer. Often it's only an indirect collision, damaging the vehicle and leaving another carcass for the roadkill crews to pick up in the morning. But the deer don't always die outright - they can lay there for hours, twitching in mortal agony. That alone is reason enough to carry a rifle in your trunk if you live in a wooded area.
I'm not sure what it says about our society when the first time a suburban kid sees an animal larger than a goldfish being killed by human hands occurs at the side of a rural highway, a farmer putting a mortally wounded doe out of its misery under a full moon. To let it just lay there and die would have been unjustifiably cruel, I suppose, but at age eight it still gave me nightmares for a week.