One day at the zoo I was admiring the gibbons, with their playfulness and charm, when a starling flew down and landed just outside their cage. At once a long arm shot out and seized it. People gasped and cried out in alarm, and then in horror, as the ape tried to pull the terrified bird through the bars.
It finally succeeded and, beating off the other gibbons, it took the starling to a clear branch and began to pull it to pieces. I can't forget the crackings and snappings, the tough white sinews, the lolling shrieking head, and most of all the curious innocent concentration of the ape.
Because, of course, the ape was innocent. It couldn't reflect on what it was doing. And if there's a spectrum running from innocence to guilt - a spectrum marked by increasing consciousness and ability to reflect - then right out at the other end of it there are the likes of Frederick and Rosemary West, who could do so.
Somewhere in between are the little killers of James Bulger. Were they truly evil, or didn't they know what they were doing? Where should we place them?
Because knowing where to place them means knowing how to judge them. Blake Morrison's troubling study of the Bulger case is profoundly concerned with these questions, and shows how difficult it is to come by any answers.
Morrison attended the trial of Robert Thompson and Jon Venables in 1993. He stayed in Preston (where the boys' trial was held) for a month, walked the route the children took on that dreadful day on Merseyside, heard the tapes of their interrogation ("Please God, never let me hear a child cry like that again. Or rather, let those who think these boys inhuman hear their all-too-human distress"), meditated on his own children and his fears for them, tried to understand.
The book echoes with references to Macbeth, and that most murder-haunted play is apt. Hardly anywhere else in literature can we learn so vividly and horribly what it is like to be a murderer, to kill and to know fully what it is that we have done.
It's not certain that Robert Thompson and Jon Venables did know, fully, and that is part of the point. Morrison is clear that putting them on trial as if they were adults was grossly inappropriate: "Childhood is a separate place...You can't lock up for life those whose lives have barely begun."
On the other hand, what they did was horrible, and it's right to lock them up for it. But again, the question of whether they could have known won't go away: "To know, and yet not know - the condition of being ten."
Morrison is very good on the appearance of things. So many of our most powerful judgements are made because of what things look like. Would Michael Howard have been able to declare that Myra Hindley should spend all her life in prison, as he did recently, without the continuing presence of that particular brutal-blonde photograph to fuel the public's loathing for her?
Part of our horror at the Bulger case is due to the video clip of the trusting toddler walking away hand-in-hand with his murderer. In this age, we can't escape these visual presences, so we must learn to read them.
Morrison brilliantly describes the look of the bleak streets and housing estates the children walked through on their way to the railway line, and is wise enough not to refrain from comment. "It must have an effect," he says; and yes, it must.
Similarly, he characterises both the appearance of the two boys and his own reactions to it, and then quotes Macbeth once more to warn himself against making judgements based on the look of things. But we must judge, because we are human, and because we are adult and responsible we must beware of the fallibility of our own judgements; but still, we must judge.
At one point I thought the book faltered. Morrison is describing himself putting his little daughter to bed, and he misleads us into thinking that we're reading a scene of seduction. I thought that I could see what he was doing with that story, but it's not the Bulger story.
There was no sexual motive in the killing; or if there was, it was never clearly established. At a first reading, this passage seemed like an error of judgement.
However, I've changed my mind. By making us complicit in a misreading, he's showing us the importance of appearances once more, and always the need to reflect, to be fully conscious.
Which leads to another presence in this book, that of the words of John Major. His statement that "We must condemn a little more, and understand a little less" is the most wicked thing any British politician has said in my lifetime. It is worse by far than Margaret Thatcher's assertion that "There is no such thing as society", which is transparent bluster by comparison.
Of course we mustn't understand less. We can't go back to being children, back to the innocence of the ape. We must go forward into deeper knowledge, painful though that is.
Morrison's honest, courageous and subtle study is an addition to our understanding, not least because it never overlooks the suffering of little James and his family. The ape's innocence made no difference to the starling.
Philip Pullman won the Carnegie Medal for children's literature last year with the novel "His Dark Materials" (Point/Scholastic)