This is a topic which I have covered in other ways before, both in the piece “Have I killed someone?” and “A Great Myth about Atheism: Hitler/Stalin/Pol Pot = Atheism = Atrocity – REDUX“. This idea that atheism causes people to do X or Y has reared its ugly head. Why am I mentioning this now? Anton Lundin-Pettersson went into a school in Sweden with a mask and helmet, looking pretty dark, and a sword, and killed two people. And it looks like he was an atheist.
PZ Myers has written about it. He did not say that Anton Lundin-Pettersson was an atheist and that this caused his killing spree, but that atheists don’t like admitting when one of their own is a bad person, that atheists pull the No True Scotsman fallacy and skew the stats on atheist atrocities.
This may well be true.
But at the same time, people invariably do not commit atrocities on account of their lack of belief in a god, or their belief that there is no god.
So what’s going on here? Well, people have a very naive understanding of causality. People think in terms of billiard balls: that one hits another which hits a third. The universe is not like that. As I wrote before in whether I was responsible for a parent helper dying on one of my school trips (I had made us really walk fast for 40 minutes on the way back, and that night one of the parent helpers had a heart attack and died):
In writing my book on free will, and in researching the Kalam Cosmological Argument, I have come to understand that causality is much more complex than one might imagine. A does not cause B which causes C in such a simplistic manner. At best, things are only ever contributory causes (see JL Mackie’s INUS notion of causality ); but even then, this assumes one can quantise time, and arbitrarily assign discrete units of existence to both events and entities.
Let’s look at the event of the class trip. Did it start when we arrived at the dockyard, when we got off the ferry, when we left, when I started organising it, or, indeed, were elements of the trip in place when I started planning the unit, given the job, got my teacher’s qualifications etc?
Of course, there is no objective answer to that. These abstract labels are subjectively assigned such that we can all disagree on them. That is, simplistically speaking, an element of conceptual nominalism. Likewise, there were necessary conditions in the parent’s life which contributed to his death: anything from his smoking, to his lack of general health, from deciding to come on the school trip, to deciding to get married and have kids. And so on.
An event happens in time and arbitrarily ascribing a beginning and an end to that event is an abstract pastime, and thus fails to be (imho) objectively and (Platonically) real.
Causality works through people, and harnessing it so that any one individual can claim themselves (morally) responsible for future effects which themselves are caused by effects preceding the individual makes for tricky philosophy. This is the battleground for the free will debate, for sure. Arbitrarily cutting causality up in such a way is problematic.
As I have set out in my analyses of the Kalam Cosmological Argument(KCA), which I hope to turn into a book (based on a university thesis I did on it), causality is not a linear affair which can be sliced and diced, It is a unitary matrix which derives from either a single beginning (like the Big Bang), something I find problematic, or eternally backward, or reaching some time commencement which could itself be a reboot. Either way, the idea of causality cannot be seen, and should not therefore be seen, in a discrete manner of units which can be attributed to equally problematic notions of events or unities. We are one big family of causality, this here universe.
So, in answer to the question, no. No, I didn’t kill anyone. Perhaps we could say that the universe did. And whatever notion “I” am, and whatever “I” am represented by, sat on or, better still, was part of the threads which cross and recross intricately and almost infinitely over each other in a mazy web of interconnected causality.
 Cause as INUS-condition. The most sophisticated version of the necessary and/or sufficient conditions approach is probably John Mackie’s analysis of causes in terms of so-called INUS conditions. Mackie suggested that a cause of some particular event is “an insufficient but non-redundant part of a condition which is itself unnecessary but sufficient for the result” (Mackie 1974: 62). Mackie called a condition of this kind an INUS condition, after the initial letters of the main words used in the definition. Thus, when experts declare a short-circuit to be the cause of fire, they “are saying in effect that the short-circuit is a condition of this sort, that it occurred, that the other conditions which, conjoined with it, form a sufficient condition were also present, and that no other sufficient condition of the house’s catching fire was present on this occasion” (Mackie  1993: 34). Thus, Mackie’s view may be expressed roughly in the following definition of ‘cause:’ an event A is the cause of an event B if A is a non-redundant part of a complex condition C, which, though sufficient, is not necessary for the effect (B). Source.
The Swedish man’s atheism is not the cause of the killing. There are myriad causal factors. Moreover, I am an atheist and I don’t go around killing people, so there have to be other things at play which lead to this. Atheism says nothing about morality, which is why frameworks such as secular humanism exist to provide a more complex philosophy than merely God not existing to define how to live one’s life.
This works both ways. Atheists have to understand that it is too simplistic to say religion causes X and Y. Often it is one of the major contributing factors, but the universe is also often very much more complex, and human psychology is very much more complex than the content of a religion causing some moral atrocity.
This is also not to invalidate the idea that something causes something else. I think we can still isolate things which are important or major contributing factors such that we can say, “X caused Y” as a shorthand for “X is one of the more major and proximal causes of Y”.