An alert reader notes this:
I found this quote
“[God] is the author of all things, evil excepted. But the natures with which He endowed His creatures accomplish a whole scheme of operations, and these too turn to His glory since it is He who created these very natures.”
The medieval church did not insist on what was termed historico-literal readings of every passage. Many were recognized as poetical, and they looked for the truth in whole sense rather than the word-for-word. This was especially so when certain reason showed that the literal sense was impossible. For example: what does "evening and morning the nth day" mean on a sphere like the earth?
In this case, the problem is "evil." In the Isaiah passage we have some poetic parallel: light/darkness and peace/evil. But the parallelism leads me to suspect the word translated as "evil" may likely mean something like "strife" or "war" rather than what we moderns mean by "evil." That is, Freddy Krueger with a hockey mask.
Evil does not exist per se. Evil is defectus boni, a defect or lacking in a good. As such, it is parasitical on the good. For example, "life" is a good. "Death" is an evil. But death cannot be conceived without life. We can easily conceive of life without death, but not of death without life, since death is defined in terms of life. Similarly, there can be health without sickness, but not sickness without health becasue "sickness" just is an impairment in heath. There can be truth without falsity, but you cannot be false without some truth to be false to. Likewise, theft depends on property, error on correctness, and so on and so forth for any evils we care to name
Since evils are not things-in-themselves, they are not created. Hence, William of Conches little aside.
In this sense "evil" really is analogous to "darkness" in the Isaiah passage. Darkness is not a thing-in-itself, either, but is simply the absence of light.