What caused the killings?
It is one thing to talk about an underlying hatred. Many people end up hating their parents but they don't shoot them. Hatred may be an underlying cause but it is not motivation. Motive, by itself, as the judge told the jury, is not enough. In his summing up, Justice Williamson pointed to the difference between motivation and intention-he gave the example of a man wanting to blow up a plane to get the insurance money. The man's motive or object-money-was clear, but to achieve his end the passengers and crew would die.
It is an extremely useful analogy. David Bain became enmeshed, I believe, to the point where he found himself in a trap. He needed to escape, but to do so he had to kill everyone.
Many will say, with Professor Trompf, that he didn't know what he was doing. This is the sympathetic view, a view shared by Professor Mullen. The crimes may have been cold-blooded but the motive(unstated) was "anything but cold-blooded." Less sympathetically, if I am right: the motive was to escape from the family, to escape from prison. To break out of prison, it is necessary to kill the guards.
In distinguishing between motive and intent, the judge was talking about ends and means, about thinking something through to achieve an end- "the means to an end," as he put it, "with the consequences along the way." Between the lines he was saying that David Bain knew exactly what he was doing.
James Mc Neish The Mask of Sanity
