It appears obvious to most people that we cannot have a crooked unless we first have a straight to measure it against. Sure, the crooked stick can exist, but the only way we know to call it crooked, instead of merely just a stick with no concept of alignment, is if we know to call it crooked because we do have a concept of a straight alignment. So if crooked exists, we must have a straight, at least in concept. We do not claim an object has a certain property unless we have a concept of that object not having the property.
The same applies with good and evil. If we have an evil act, the the only way we know to call it evil is to have, at least in concept, a concept of the act not being evil, which is good. So our critics who quickly point to objects in the world as being objectively, universally evil are actually doing so on the basis that objective, universal good exists. This reasoning is one of Aquinas’ five ways.