It takes nothing to get a can of spray paint and deface a wall with an insult or any crude depiction. Imagine the commitment required to carve an insult on stone. This man was truly angry.
Your surmise depends on whether some random provincial soldier at the shit-end of the empire would really be socially superior to a skilled craftsman, that this was the work of a skilled craftsman, that the soldier could afford the sculptor's commission, that this sort of act really was delegated sometimes, and finally, that he wasn't truly angry because he could afford to hire out the curse?
Or -- said soldier had status to compel some aliquis to inscribe an insult and therefore wasn't really angry? Your idea sounds like an aborted Plautus plot.
To paraphrase a Brian (not the one mentioned in the top comment), I hope you stood up before pulling this out of your ass. Pardon me while I scrape something into the nearest concrete.
An enormous amount of what is attributed to famous authors was written or compiled by their slaves.
I take the serifs as evidence for the surmise. Even a slave can have pride.
The sort of literate slave you are suggesting, BTW, would be fit for a patrician and command a price beyond what a provincial soldier could afford. Of what use would such a slave be to such a soldier anyway?
You're really stretching it to preserve this fantasy of yours.
Curses, especially, were routinely farmed out. Executing curses oneself was considered dangerous, like practicing law today. Priests were consulted. Like today.
Very, very often -- indeed, absolutely conventionally -- literate slaves were kept as teachers. Any soldier with aspirations to advancement would need education.
You are really stretching to avoid trivially obvious inferences. Read some history? Maybe a literate slave could help you out.
Advancement could be yours!
Take a step back and look at the claim and counter-claim: guy must have been mad to carve that in stone; nu-uh, could have been delegated. I've given many reasons to doubt this is likely. Now you've got "priests" in the mix (this is suddenly a state function, I guess) to hold up this house of cards? The chain of contingencies your "remarkably obvious inference" depends upon is remarkable. And you are inferring all this from a single piece of epigraphic evidence? Good grief.
Ignorance, today, is a choice. You may dispense with it anytime you like. Or not; "nescio id quod sciam," as it were.
Why is it so unlikely that the aggrieved person carved this himself?
Carving it himself -- or herself, we don't have any indication who, or that it was even a soldier who was aggrieved -- is one of many possibilities.
What is ignorant is to insist only the one alternative, among many, is possible, and that slaves were never ordered to do tedious chores, and that a soldier (or whoever) could not have had a slave, and that said slave could not be literate. All officious BS: what we have is a rock. What we know is, exactly, that somebody carved it.