If the strongest people are unwilling to work together and cooperate, and would, instead rather backstab each other, in hopes of obtaining an opportunity, then those strongest people are part of the enemy, rather than your ally. In fighting during a war will bring devastating casualties. The MC lived through this personally.
You saw how the two groups that supported the 1st and 2nd princes were supposedly fighting together against the McClains, only to turn on each other before they even reached their goal, and before the sword duke arrived. That’s exactly what would happen if you left them alive with the third prince as the king.
Those greedy individuals would look to assassinate the 3rd prince. With no remaining royalty, the throne would be up for grabs. Either that, or they’d try to assassinate each other, in hopes of climbing the ladder of power, under whichever prince became king.
The problem lies in their willingness to betray someone they are in an alliance with, for the sake of their own goals. They wanted to put their chosen prince on the throne, not because their prince was worthy or capable, but because the prince was their blood relative and that would give them power.
The ones who swore allegiance to those princes actually were swearing allegiance to the prince’s powerful blood relatives, who they knew, would reward them for that allegiance. Everyone’s motivation was greed. That’s a huge problem.
Also, nobody but us readers and the killer, know that both princes are dead. The kingdom only know that one prince is dead, and that the other one is missing. From what we saw ant the ending of the last chapter, we can conclude that the man who killed the other prince was a spy from the enemy nation. Therefore, I highly doubt that he reported back to anyone (other than the enemy nation) that he killed the missing prince. That means all the people who were still supporting that missing prince were gonna keep causing problems if they were still alive.
Half the country didn't get wiped, just the nobles connected to the two princes and the princes' lines themselves.
Unless you want to deal with them regrouping and causing constant disruption when an enemy nation is breathing down your neck, you kill them. If you can't get the prince, you wipe out his support system so you can at least buy enough time to solidify your reign.
Nobles are few in number. It's easy to redistribute the wealth of fallen houses to allies. The actual population is unaffected by a few dead nobles.
Historically, in the real world, whenever the descendants of royalty were spared during a revolution, people who lost power would gather together and declare that descendent as having the right to inherit the throne. Then they’d try and take over the kingdom using that as their justification. The descendent would become a kind of banner for the greedy to gather under.
It’s happened so many times throughout human history, that’s the reason why even children had to be killed, if they were the descendants of royalty. Doesn’t matter if they weren’t direct descendants. If they had any claim to the throne, even distant, they had to die.
He killed the supporters, AND the two princes’ offspring. That was written in the series.
As for why he would kill the supporters, it’s because those supporters don’t care about the kingdom. They care about the power they can obtain, even if they have to sacrifice the well-being of the kingdom in order to obtain it. That makes those individuals a threat to the kingdom. They are not the kingdom’s strength, they are the kingdom’s weakness.
If they would bow down to an incompetent prince, out of greed, and wage war against their own countrymen, out of ambition, then what’s to stop them from siding with the empire when it shows up and tries to take over the kingdom? The MC knows the empire is going to take over. And he knows these families are the ones who will willingly sell out anyone to satisfy their greed.
Why would anyone keep future traitors amongst their ranks, while facing an opponent that those traitors might swear loyalty to out of greed and ambition? Remember, our MC knows the future. He knows exactly who gave into the empire. He knows exactly who will switch sides to the empire. He knows exactly who is a threat to the kingdom.
So more than anything, my question is: Why are you questioning the MC’s decisions about who is the trader and who isn’t, when he knows the future, and wants to save the kingdom from that future? We already know he’s someone who will act based on the information he has, due to time travel, for the well-being of the kingdom’s future. I feel like you’re ignoring the fact that the MC is a regressor…
While there are a lot of nobles in this series with major combat power, those nobles wouldn’t have sat back and let allowed themselves to be slaughtered so helplessly. The ones who were killed were the nobles who only had political power, not combat power.
The combat power of the executed nobles, was the knights they employed. However, if you only kill off the nobles, without executing their knights, then the knights can move on to other noble families, and be employed there. So we haven’t actually lost our combat power by getting rid of our political weaknesses.
Considering we saw the faces of all the ones who were executed, and that small group didn’t include entire squadrons of knights, I feel like you might be getting ahead of yourself in the assumption department. (〜 ̄△ ̄)〜
Girl they were fighting him because he was openly interfering with their little fight to put their chosen (at the time, still living) prince on the throne. Yeah, no shit they’re gonna try to align to take him out. At no point were they trying to destroy the kingdom. The only one selectively reading here is the person who somehow missed the entire point of the last 40 frickin chapters lmao.
Ronan McLain did not attack other noble families, during the first 40 chapters, without a good reason. During his first life, the nobles in the surrounding lands attacked his family without just cause, and without following proper legal procedure.
Those nobles declared a fief war against the McLain family. They did not get royal approval for this fief war. They lacked sufficient cause for their personal war and they knew that. They also did not provide adequate time between their declaration of war and their arrival, which is part of noble laws. During the fief war, those nobles killed his father. Afterwards they claimed some of the McLain territory as spoils of war.
Why did they want McLain territory? It’s because those nobles had discovered a gold mine on McLain’s territory. Those greedy nobles knew about the gold mine before the McLain's themselves became aware of it. As for which nobles were involved in it, there were 3 separate noble households.
So yeah, the first 40 chapters are about Ronan McLain defending his family and territory from criminals, who defiled royal Law in order to obtain a goldmine that wasn’t rightfully theirs.
But sure, you’re not a selective reader, and we’re the ones who missed the point of the first 40 chapters. Got it.

Okay but if he’s trying to keep his country strong enough to resist the Empire invasion why is he killing all the strongest people.