It shows that he doesn't have empathy, or a passion to help others. He's a psychopath who will kill those that annoy him or that he can gain from.
Why should morality matter to him? He's spent enough time working for those in the world, only for him to have to do it again, even after 10 years.
Why
shouldn't he kill all of them? Killing those that he did allowed for the major problems of the world to be solved. The small ones, like lost cats, thieves, side quests, and gathering quests shouldn't really matter, no?
Why is he a psychopath? He knew his goals, and followed them to a T. He did it in (nearly) the fastest possible time, and achieved the end goal. Well, thinking about that one a little more may make your point hold true, but to that I say, why should it be a negative thing? Surely, after reading so many manga with similar storylines, it is obvious how being 'right and just' never really works out, and always leads to psychopathy to an extent, anyway. There's always a point in which the hero 'goes mad with rage' etc etc.
What is the point in being kind and fair in the face of those who do not care for others, either? He killed a racist ruler who was to die, anyway, nipped corruption in the bud and gave power to those pure of heart, gave no slack to the rich and royal, and brought order to the place he came from. Why is this being ignored? For what, being ruthless in how he did it? If done the 'proper way', many who were executed would have either 'gotten away' with their crimes, or only been imprisoned. Setting the precedence that there is no real punishment for crimes as the manga describes only allows for lenience, which, in turn, only leads to ignorance and so further perpetuation of said people. As a consequence of what the hero did, other nations began to deal with their own problems, and even feared the hero, for they knew what would happen to them if they allowed him to intervene.
It's not really his fault if others don't like his methods. Sure, they are questionable, but they worked, no? Bear in mind that this isn't real, nor is it realistic, nor will it ever happen. Questioning morality isn't really necessary here.
At this point, what he did was the lesser of two evils, and is (somewhat) justified, in my opinion. If he didn't sweep up the kingdom in the speed that he did, then the corruption and malice would have continued for a further
6+ years. That is an awfully long time, especially in how people were being sacrificed for the demonic prayers, as well as for the nobles to level up.
Then again, even in saying all that, would I do the same as what he did? Who knows, who cares? I certainly don't.
Off to the next summon hero/reincarnate manga, I guess. Actually, maybe another cultivation manga? Maybe number 34 will be different from the last 33...
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PS: if you still don't get my poor analysis/what I talked about, go read Iron Ladies. It's a more in-depth example of what this hero did, except it goes into more detail, and MC explains their reasoning more. It's a lot better, too.
Here's the link: https://mangadex.org/title/32020/iron-ladies
Edit: I forgot to add a line.