Why is God's moral neutrality the proper conclusion to draw from the evidence? Philo tells us that it is because there are four possible hypotheses about the moral nature of the first cause of the universe, and all but one of them can be eliminated. But he does not go into much detail about this surprising claim, and so it will be helpful to go through it ourselves.
These are the four possible hypotheses about the moral nature of God: (1) God is perfectly good. This, naturally, is what the empirical theist wants to conclude from the evidence. (2) God is perfectly malicious. Almost no one would want to hold this view. (3) God is both good and malicious. (Or, alternatively, there are two different forces in the world, one good and one evil). Several sects have believed in a picture like this one, most notably the Manichæans cited by Philo. And finally(4) God is neither good nor malicious.
Given the mixed nature of our world and assuming that God could take away or augment suffering, the first two possibilities are immediately ruled out. We cannot infer a perfectly good God from a world that contains evil, and we cannot infer a perfectly evil God from a world that contains good. The reason that the evidence does not support the third possibility is more difficult to understand. It would seem that a mixed world like ours points precisely to some dueling moral forces, such as God and Satan. This inference certainly seemed strong to many cultures which posited just such a picture. Nearly all ancient mythologies make use of this sort of explanation to account for the mixture of blessings and curses in our life, and there are numerous sects in the Judeo-Christian tradition that uphold this view as one of their central tenets.
Philo points out, though, that our evidence does not point to this picture at all. Good and evil in our universe follow general laws. There is no indication of any struggle. If there were a struggle taking place (between two beings, or between two sides of God's nature) then we should expect to see sudden we should expect to sometimes see miracles performed to benefit the good, and other times see feats performed to help out the evil. Instead what we see is that our world simply operates based on immutable laws of nature, and whether one is harmed by them or benefited by them has nothing to do with how good or evil one is. Bad things happen to good people and bad things happen to bad people, but (as far as we can tell from the evidence) only because both good people and bad people are subject to the laws of nature.
All we are left to conclude, then, is that God is indifferent to good and evil. The laws that he set up affect the good and the evil exactly alike because they were not set up with any moral preferences in mind. Certainly, we might think that if God were perfectly good he would have at the very least set up laws of nature to benefit the good, but he did not even do this. So far as we should trust the evidence, then, we should conclude that God is morally neutral.
The next obvious question to ask is how far we should trust the evidence. Philo claims that we should not trust the evidence at all: instead we should rely on revelation to tell us what God is like. But Hume might not have shared Philo's view on this issue. As a serious empiricist he believed that all we should trust in is the evidence. The wise man, he says in the Enquiry, is the man who proportions his belief to the evidence; in other words, what it means to be wise is to try to believe only what you have evidence to believe.