Let’s start with what generally causes harm. Physical and/or psychological. What you are actually saying to me is that you could not, without reference to God, come up with a reasonable argument that says that burning children is wrong. I don’t believe that for one minute. It’s as simple as me asking you if you think it’s wrong to cause someone physical harm for your own pleasure.
If you agree that it’s wrong, then we have a basis for deciding what is right and what is wrong. If you disagree, then the technical term for you would be a psychopath.
I strongly suspect that what you are doing here is burying your argument in the assumptions of those who hold a view opposite to yours.
Time to unravel your grounds for believing something to be “wrong” or, even, “harmful,” since that is where you choose to ground your case.
If God does not exist and, the implication surely follows, the mindless, unguided vexations of atomic and subatomic particles are all that are left to us to ground whatever “deep” thinking is left to us, that means there is NO ground in the purely physical universe to moor or substantiate the meaning of “harm.” Sure some reactions in the material universe of violent and those reactions often engender the disintegration or dissolution of entities, but also in the originating of new entities. The strictly material universe does not recognize “harm,” even when what appear to be profoundly beautiful or valuable entities are irreparably terminated.
To meaningfully apply the word “harm” positively requires the presumption that the entity that was “harmed” OUGHT not to have been and there is something profoundly wrong in that “harm” being brought about.
In a an eliminative and materialistic universe, there are NO grounds for a claim of harm other than from a tenuous position of “relative” to some subject or other. This, however, creates another problem. There are some subjects who feel or think that harming others greatly benefits them - ergo, they presume that the violent dissolution of others, materialistically speaking, is nothing more than the kind of disintegration or termination of entities that occurs chronically in the larger universe. The fact that you want to insist such individuals are psychopaths or deranged is really of no consequence to them, since their ground for committing what you feel are heinous acts is found in the very nature of the material universe - things go out of and come into existence all the time with nary a notice on the part of the atomic and subatomic particles that are the ultimate constituents and governors of the universe itself - if atheism is true.
In other words, you have NO grounds in materialism to claim your view of “harm” to be any more a grounding moral principle than a less scrupulous materialist has for dismissing morality altogether. Whereas you arbitrarily choose to recognize “harm” as an idea you wish to invoke to prevent yourself from engaging in certain behaviors, the less inhibited materialist could - with just as much justification in materialism - claim that is merely a hangup of yours (your disparaging references to psychopathology notwithstanding.
Note that the theist has very different - and far more tenable - grounds for invoking harm than you do. For the theist, existents have purpose and their value and meaning is determined by that purpose which is built into the very fabric of existence itself, Being Itself - Ipsum Esse Subsistens.
You can dismiss that notion as mere superstition, but, logically speaking, that dismissal undermines any case you could possibly make for “harm” as grounds for your system of morality.
Your argument relies heavily on an appeal to the consistency of morality as grounded in theism, but fall apart - is irrevocably harmed - by your materialism and atheistic presumptions. In other words, your inconsistency is must remain hidden behind the very premises the theist grants merely by accepting theism but become glaring if viewed from the atheist materialism on which you want your argument from “harm” to stand.