HOLD
THE FUCK
UP.
MC's "mother and father married for political reasons" according to this very series.
That means his Mother AND his Father held a relatively HIGH RANK in their country's nobility.
You don't just kill a Duke's daughter, nor do you simply pretend that the rightful heir of a landed noble "disappeared," and start calling them a fucking commoner!
NO
FUCKING
WAY
Seriously; if don't know if I can continue with this bullshit.... I am all about suspension of disbelief; can't even read an isekai if you're not willing to turn off at least some of the logic centers of your brain!
So, I get reading impossible bullshit before breakfast; no problem with that.
BUT.
But, but, BUT!
If there is some form of social strata, some kind of bullshit, lying belief in your blood/mana/essence being inherently "better" than other people, then the other people who believe that will NEVER EVER EVER just pretend that one of their own "better" people is "just gone." They might accept it, and MC's family might get away with it if it is a "third son," or some "fifth daughter" who "disappears," but not the LEGAL HEIR who's parents were Specifically Forced To Mate (arranged marriage) in order to Produce An Heir!
The only reason you arrange a marriage is to cause two specific people to have a child; might be a favor, might inherit larger lands, maybe you want a kid with 6 toes; whatever. You ONLY arrange marriages to increase power; family, political, personal; merging lands, support, or funds... and then you raise the child of that marriage, and ONLY FROM THAT MARRIAGE, to inherit all of that power.
That's it; family gain, personal debt, and power.
ONLY
REASONS
These are facts of being human, with a brain that works as a human brain, and are required for us to be able to understand this story.
If these facts are not true, then there is NO REASON for him to take revenge!
If there is literally only the Great Boomer Reason of "Because I Said So," and that rules all of society... then just kill the nobles, or merchants, or the king, or anybody you want; there are no actual rules in this society, and the MC should be happy about his situation... because that's what a violent anarchy looks like, and that's the society being described.
If this ISN'T a violent anarchic society (and it'snot; there are merchants, and that alone precludes the possibility), then that means that there are rules... and that means he ISN'T a commoner, no matter what his father and illegitimate siblings try to say.
Which means the entire basis of every interaction makes absolutely No Sense.