How can people even dare to judge the main character as a psychotic arsehole?
He is but the product of the human abyss present in front of him. Every "psychosis" is the direct result of someone abusing him, be it physically, mentally or emotionally.
Compared to that torture, what he does in revenge is basically Euthanasia.
And before you go out of your way to call people who use the term Euthanasia mental/fascist/whatever, you should look at the actual meaning related to the term on the base of a society.
People should read both Friedrich Nietzsches "Also sprach Zarathustra" and Jean-Jacques Rousseaus "The Social Contract" to understand the true meaning of societies and social behaviour.
It clears up the misconceptions people have about "death sentence".
It's not mindless killing as stated beneath by someone who seems to be too deeply influenced by nowadays political correctness.
We're talking about the death sentence as a form of "relieve killing" out of sympathy with the assailant [compare Nietsches "Also sprach Zarathustra" first book, 6th speech "Vom bleichen Verbrecher"] because said assailant is no longer able to suppress his insanity, taking over his rationality.
Thus he became the enemy ["he should be called fiend/enemy, not assailant" by the judges] of himself and humanity.
Nietsche states the essence in these sentences : " There is no redemption for whom, who is suffering so from himself, other than a fast death.Your killings, you judges, should be sympathy and not revenge. And through your killing, you yourselves shall legitimate life."
Basically, what is happening here is the confrontation between human cruelty, justice and "social sanity".
As Rousseau states in his work "The Social Contract", life and death in a society are based on the cooperation of every member of the state. This ensures the security and life of every member of society, and enables that nowadays societies are so much more differentiated than then rudimentary societies during the late stone age.
However, this also makes the reverse statement true: Nobody in the state community would be alive if it were not for the cooperation of the other members. This is why death sentence is not a moral atrocity but rather the complete excommunication of the person who does no longer want to be part of the society.
As his life is a product of the society he denies through his actions against the society, he denied the source that gave him life and which supports his life to the moment he dies.
In other words, death penalty is self-induced social suicide [as the saying from the bible states " the one who lives by the sword dies through it" also holds true for the society "those who live through society die through it"].
So, in the case of the main character of this story, we are talking about mercy killings out of sympathy with the victim (as they are all twisted to the point of no return, harming society) that also go in line with the self-denial of his victims as a part of society [and thus their lives] through their actions that undermine the very reason they are alive the moment that they are supposedly suffering through "torments" by the main character.
And while the main character seem to have been very much mentally afflicted, he still has more than enough intact rationality to not target those who are not denying their very existence by attempting to rob/abuse/kill him, who is an outsider that has nothing to do with them.
This is clearly not some "mindless killing". The world he is summed into is warped to the point that so many people are denying their own existence it's a sheer wonder society hasn't already collapsed.
Maybe that's the real reason behind these standard tropes in Isekai-genre literature that require the summoned hero(es) to subdue a demon lord, who basically only is able to attack humanity because their society is some mindless Utopia that isn't handling the people who deny the society correctly, thus resulting in a near-collapse state that enables the demon lord to attack and win in the first place.
All in all it waters down to "homo himini lupus", and the rest is just the law of cause and effect applying accordingly to the situation.