By gratuitous I mean 'easily preventable harm, yet allowed to happen to the victim due to callous disregard or enjoyment' — 180 Proof

Is this gratuitous state necessary in order for something to be evil?

"Relative to the perpetrator's value system"? You mean like "just following orders"? — 180 Proof

Following orders could be an example of -- but not sufficiently so -- of one acting to their value system. I mean to say, that this person is performing this harmful act in accordance to their conceptual beliefs, or value system. For example, a man is killing people as to release them from what he believes from a world of suffering. It is his belief that he will stay here in this world to suffer, he would off himself, but then he wouldn't be able to "help" others through his benevolent slayings. To the other, his act is evil, but to him he is doing the world good.

Think. Doing evil – producing corpses or dehumanized automatons/beasts – is not as subjective as that; the more unthinking, the easier it is to excuse and to do it. — 180 Proof

I'm not quite sure what you're saying, would you elaborate on this?

In a nonreligious / secular context, or discourse, evil amounts to ... indifference to, or infliction of, gratuitous harm that culminates in destroying moral agency. — 180 Proof

Is it necessary for this harm to be done in a gratuitous fashion? One could also consider the relativity of a value system to another. That is to say, that this act of harm could be done with good reason relative to the perpetrator's value system.

I think the struggle here is the aim for an Absolute. What I offer instead is to ask another question and consider its implications:

Do I control my breathing, or does my body?

In order to have a dimension of evil, we first have to create a framework that gives image to it. This framework is a value system. Without reasoning capability (cognition) we have no home for metaphysical concepts like evil. For example, I love A, if A were to be negated by some other, I now consider this other as approaching evil to the degree at which they have negated my value A.