Genesis 4 contains the story of Cain and Abel. In the light of world history, it is a foundational text when it comes to understanding the nature of human beings. It sheds a cold light on what happened yesterday in New Zealand, when at least 49 Muslims were gunned down by a terrorist who appears to have been a right-wing extremist.

In the story, God looks with favour on Abel's offering, but not on Cain's. No reason is given; that's just how things are.

Very often there is no apparent reason why one person succeeds in life and another fails. The question is what we do with our failures. God says to Cain: 'Why are you angry? Why is your face downcast? If you do what is right, will you not be accepted? But if you do not do what is right, sin is crouching at the door; it desires to have you, but you must master it' (6-7).

We don't know what led the Christchurch killer to do what he did. At one level it's important to find out; at another, not so much. Because the Bible lays out the path to murder very precisely. There is a movement towards evil, an opening of the self to wrongdoing. Then 'sin' – personified as a sort of beast, waiting in ambush – pounces, and there is a struggle between the desire to do right and the desire to do wrong. 'You must master it', God says, but by then it is often too late. It was for Cain; it was for yesterday's murderer too. It mastered him.

What, for him, was the equivalent of a rejected sacrifice? A grudge against others he thought were more successful than him? A fear of those he saw as 'other'? A twisted loyalty to his own race or religion?

What led him to walk towards the doorway into hell, where sin was crouching? Hate groups on social media, far-right conspiracy theories, drugs, bad company?

We might find out one day. But what's more important than satisfying our curiosity is recognising the power of sin. It draws us where we don't want to go, until we find we do want to go there, and then it has mastered us.

Anyone with a heart is grieved at what happened in Christchurch. But if we take from it the thought that there are a few unpredictable aggressors who will break out and kill, and that there's nothing to be done about it because they are such outliers, we've learned the wrong lesson. The ancient story of Cain and Abel tells us that murder is the ultimate consequence of a movement towards evil, and that this movement is one we're all capable of making. Islamophobia, racism and resentment are the claws of the beast. The beast will be there whatever we do, but its claws can be blunted. We all have a part to play in this: we can challenge the throwaway comment, move towards our Muslim neighbour, speak words of peace.

It is too late for those who died yesterday. But we're called to do what we can, and most of us should probably do more than we do.

Follow Mark Woods on Twitter: @RevMarkWoods