Evil is a lack, not an attribute. We as shorthand like to say it is an attribute, but that is all this is: shorthand, a metaphor. Here we need be precise. Next week some purported counter-examples to this assertion are given, along with their rebuttals.
1 From these considerations it becomes evident that no essence is evil in itself.
2 In fact, evil is simply a privation of something which a subject is entitled by its origin to possess and which it ought to have, as we have said. Such is the meaning of the word “evil” among all men. Now, privation is not an essence; it is, rather, a negation in a substance. Therefore, evil is not an essence in things.
3 Again, each thing has actual being in accord with its essence. To the extent that it possesses being, it has something good; for, if good is that which all desire, then being itself must be called a good, because all desire to be. As a consequence, then, each thing is good because it possesses actual being. Now, good and evil are contraries. So, nothing is evil by virtue of the fact that it has essence. Therefore, no essence is evil.
Notes It is hard to believe this in the case of mosquitoes, but there you are. (Joke.)
4 Besides, everything is either an agent or a thing that is made. Now, evil cannot be an agent, because whatever acts does so inasmuch as it is actually existent and perfect. Similarly, it cannot be a thing that is made, for the termination of every process of generation is a form, and a good thing. Therefore, nothing is evil by virtue of its essence.
5 Moreover, nothing tends toward its contrary, for each thing inclines to what is like and suitable to itself. Now, every being intends a good, when it is acting, as has been proved. Therefore, no being, as being, is evil.
6 Furthermore, every essence belongs to some definite thing in nature. Indeed, if it falls in the genus of substance, it is the very nature of the thing. However, if it is in the genus of accident, it must be caused by the principles of some substance, and thus it will be natural to this substance, though perhaps it may not be natural to another substance. For example, heat is natural to fire, though it may not be natural to water. Now, what is evil in itself can not be natural to anything. For it is of the very definition of evil that it be a privation of that which is to be in a subject by virtue of its natural origin, and which should be in it. So, evil cannot be natural to any subject, since it is a privation of what is natural. Consequently, whatever is present naturally in something is a good for it, and it is evil if the thing lacks it. Therefore, no essence is evil in itself.
7 Again, whatever possesses an essence is either a form itself, or has a form. In fact, every being is placed in a genus or species through a form. Now, a form, as such, has the essential character of goodness, because a form is a principle of action; so, too, does the end to which every agent looks; and so also does the action whereby each thing having a form is perfected. Hence, everything that has an essence is, by virtue of that fact, a good thing. Therefore, evil has no essence.
8 Besides, being is divided by act and potency. Now, act, as such, is good, for something is perfect to the extent that it is in act. Potency, too, is a good thing, for potency tends toward act, as appears in every instance of change. Moreover, potency is also proportionate to act and not contrary to it. It belongs in the same genus with act; privation does not belong to it, except accidentally. So, everything that exists, whatever the mode of its existence, is a good thing to the extent that it is a being. Therefore, evil does not possess any essence.
9 Moreover, we have proved in Book Two of this work  that every act of being, whatever its type may be, comes from God. And we have shown in Book One [28, 41] that God is perfect goodness. Now, since evil could not be the product of a good thing, it is impossible for any being, as a being, to be evil.
10 This is why Genesis (1:31) states: “God saw all the things that He had made, and they were very good”; and Ecclesiastes (3:11): “He hath made all things good in their time”; and also I Timothy (4:4): “Every creature of God is good.”
11 And Dionysius, in chapter four of On the Divine Names says that “evil is not an existing thing,” that is, in itself; “nor is it something among things that have existence,” but it is a sort of accident, something like whiteness or blackness.
12 Through this consideration, the error of the Manicheans is refuted, for they claimed that some things are evil in their very natures.