Two important milestones: a definition of science and evil.
…Again. Since science is the knowledge of a thing by its proper cause, and since He knows the order of all causes and effects, as we have shown above, it is evident that science is properly in Him: not that science however which is caused by reasoning, as our science is caused by a demonstration. Hence it is written (1 Kings ii. 3): The Lord is a God of all knowledge…
Notes The definition of science comes from The Philosopher, in his Posterior Analytics. Notice that this is not the modern definition, which pertains mainly to predictive ability. Knowledge of cause sometimes comes along for the ride in the predictive equations scientists develop, but this happenstance is not quite by design. Unfortunately, many scientists think their semi-determinative equations prove cause. But regular readers know this isn’t so; new readers can click Classic Posts and look for asinine uses of statistics.
 FROM what has been said it can be proved that God cannot will evil.
 For the virtue of a thing is that by which one produces a good work. Now every work of God is a work of virtue, since His virtue is His essence, as we have shown above. Therefore He cannot will evil.
 Again. The will never tends towards evil unless there be an error in the reason, at least as regards the particular object of choice. For since the object of the will is an apprehended good, the will cannot tend towards an evil unless, in some way, it is proposed to it as a good; and this cannot be without an error. Now there can be no error in the divine knowledge, as we have shown. Therefore God’s will cannot tend to evil.
Notes And that’s sin, sisters and brothers. Choosing what the will thought was a good in error. That makes sin objective, not relative. This is why unreflective appeals to one’s conscience as insufficient to knowing the good or evil of a thing. Conscience is not doing what feels good or right in the moment; rather, it is much more than that. Here’s an analogy. Think of solving high school math “story” problems. You can certainly answer using your gut, but it’s far better to learn the rules, and they whys of the rules, of right and wrong.
 Moreover. God is the sovereign good, as was proved above. Now the sovereign good does not suffer the company of evil, as neither does the supremely hot suffer an admixture of cold. Therefore the divine will cannot be inclined to evil.
Notes Homework problem: What other metaphors, besides admixtures of cold and hot, might work here?
 Further. Since good has the aspect of end, evil cannot be an object of the will except the latter turn away from its end. But the divine will cannot turn away from its end, because He cannot will anything except by willing Himself, as we have proved. Therefore He cannot will evil.
 It is therefore evident that in Him free-will is naturally established in good.
 …Hereby is confuted the error of the Jews who assert in the Talmud that God sins sometimes and is cleansed from sin; and also of the Luciferiani who say that God sinned in casting out Lucifer.
Notes The first one’s for you, Ianto. I hadn’t known anything about the Luciferiani, but this Wiki article might be of help.
Evil exists, of course, and can be chosen by us, but only because of error, only because something we thought was good was, in fact, objectively the absence of good.