At what point in human evolution did it become a crime to kill someone who annoys you? We cannot know. We can only observe that in other species the pros and cons of killing are weighed on a purely practical basis.
Carnivorous animals choose prey that is likely to go down easily. They're looking for a good meal, not a good fight. Animals that battle for breeding dominance might not kill each other outright, but might leave both winners and losers too drained to make it through the winter. Sometimes, fatal injuries occur in the ritualistic confrontations as well. There seems to be no social stigma in the herd. It's just how their lives work.
Humans are more complicated than the more specialized species. We can exhibit aspects of every other life form from algae on up. No one has adequately catalogued the infinite variety. This makes it hard to write a good rule book. Even the concept of a rule book is unnatural. We have evolved some widespread general principles, which usually include a disapproving view of murder. But in the back of most minds, and much nearer the front of some remains the memory that at one time you could be more impulsively violent and come out better rather than worse for it, if you won the confrontation.
The gunslinger libertarian view of humanity holds that the only way to keep people in line is with the threat of violent retribution. An aggressor must be supported, if the cause is worthy, or opposed by force. In a world where everything is either a metaphor for war or an actual war, nothing prospers for very long, but anything that wins has earned its place at the top -- however brief -- by superior force. Eat the best food. Claim all the breeding females. The nostalgia runs so deep that it is prehistoric.
As civilization breaks down under all of the accumulating pressures, our advanced ability to rationalize and fantasize teams up with primitive instinct. We haven't quite reached the point where you can kill someone and go on about your day without further interruption, but tempers seem to be growing shorter. How often do you find yourself wondering whether you should speak up about something you see, because the person you see doing it might just pull a gun on you?
"Hey! Pick up your trash!"
Blam blam blam blam!
Authoritarians also believe that when all else fails, unruly citizens should be held down by force. There's less freelance murder, but the regime's scapegoats are fair game. And there are lots of job opportunities in government service for loyal violent people with no conscience. Under an authoritarian regime, the preliminaries to violence will be repressive laws guaranteed to irritate someone sooner or later. And then it's time put down the uprising, whether that's a single, loud dissenting voice, or a gathering of aggrieved citizens in public protest.
So far, there are only a few ways to get away with murder. Killing a bicyclist on the road is practically a freebie. Pedestrians are almost as easy. On a lonely road, just hit and run. In the presence of witnesses, you must stop and express remorse to avoid facing legal action. Make it look good! Otherwise, your legal defense might cost you a few thousand dollars and a couple of missed days at work.
In states with "stand your ground" laws, the person who shoots most accurately generally gets to tell the story. Your odds are much better running over a cyclist, because the use of a gun is more likely to put you in front of a jury. Not only that, most road users are just as happy to have one less cyclist in the way. A lot of people will be sympathetic to the poor motorist who suffered the trauma of snuffing out a reckless idiot who insisted on wobbling around on two wheels among the big, dangerous vehicles.