Here's a recurring dream I used to have growing up on the South Side of Chicago. I'm walking down a street by myself and a man is walking toward me. Just as we're about to pass each other, I see that he's holding a razor, a razor that he lifts quickly to try and slash across my face. I jerk my head back violently, which is the moment that wakes me each time – breathless, my heart shaking my chest.

I had this dream in response to a series of real events. For a short while during the 1950s, a man was terrorizing women and girls in public places in Chicago, randomly slashing at them. But I went on having the dream, in one variation or another, well into my forties, I think because the crime seemed so terrifyingly random, a matter of pure bad luck. Like going to the movies and having someone step out onto the proscenium in battle dress and start shooting. Like sitting in a classroom learning to read when the door opens and a madmen enters, fully armed.

Now here comes the summer of 2014, and with it the statistically inevitable rise in violent crime and murder. Caused at least in part, we're told, by the temperature. It gets hotter, irritability increases, we all head outside, bumping up against each other, pissing each other off.

And already, with summer still weeks away, the carnage has begun. A man in New York goes on a stabbing spree that includes two little children. A man in California knifes three people, then shoots and kills three others before killing himself. A man in Seattle walks into a college, a man in Georgia walks into a courtroom, both of them equipped for and apparently willing to kill as many people as they possibly can. A white supremacist couple in Las Vegas. Just Tuesday, a lone shooter in a high school in Oregon.

Is it random? Is it the forecast? Or is this kind of violence now as inevitable as it is unpredictable?

I thought of my violent dream again decades later, when I read about the murder of a young foreign student at MIT. He was walking with a friend on a warm September night when they passed some kids from the local high school. The kids were drunk, high, and one of them, for no apparent reason, punched him so hard in the forehead that he fell down. While he was trying to get up, a second kid stabbed him in the heart, then wiped off the bloody knife on his shirt as he lay there dying.

The Cambridge event itself was awful enough, but what struck me in its aftermath were the comments made to reporters by friends of the killer. They were of the opinion that entirely too much was being made of this murder. This kind of thing happened all the time – what was the big deal? The quote in local paper from one of them was, "People get killed every day".

Technically, of course, this is true. And while the year Yngve Raustein was punched and stabbed and died, 1992, was a bad year for homicide in the United States, it wasn't so bad in Cambridge. There were all of two murders in Cambridge. It was worse in the bigger city across the river, but even in Boston, there were 73 murders. I hesitate to say "only" of either number – 2 or 73, both are too many – but what I can say is that these kids had no direct experience of people getting killed every day. Not even close.

Still, all around us that notion – that killing is everywhere, and so frequent as to be ordinary – is nourished. By certain violent video games, by slasher and torture movies, by graphic crime shows on television, by the internet with its celebration of the grisly, the lurid. I don't know enough to make the direct case that any of these might actually increase the probability of violent crimes, but statistics indicate that those who watch a lot of crime shows have an unrealistic and distorted view about the frequency of those crimes: they think they happen all the time. And it seems to me if you accept killing as no big deal, then you live in a world where killing may seem a reasonable answer to some sense of generalized insult, or deprivation, or racial hatred, or paranoia, or political injustice – or even just boredom.

What happens next in that kind of world? What other random horrors will a long, hot summer bring?

Perhaps, as it turns out, none. Because the moments of terrifying violence we've seen in the summer of 2014 are precisely the kinds of crimes notincreased by hot weather, by proximity, by opportunity – those elements that seem to be the drivers of the seasonal spike. These crimes are unpredictable, in some sense impersonal, apparently driven by mental illness and sometimes made worse by the terrifyingly easy availability of guns. No, it is only interpersonal crimes – crimes of passion, of aggression, that will get worse over the next few months. The risk of being killed or injured by someone you don’t know, someone with a semiautomatic, with a grenade, with a knife, with a straight razor – this stays about the same year-round.

Lucky, aren't we?