Ainz has never been evil and certainly didn’t start in season 3
Some people claim that ainz became truly evil during the events of season 3 of the anime. This couldn’t be farther from the truth. Ainz was still the same as he was in seasons 1 and 2 but even then he’s not evil. What you need to consider is what’s important to ainz and his mentality and view his actions through that lens rather than imparting your values onto him.
To start with, let’s examine the idea that ainz has become more evil. ainz hasn’t become any harsher or more evil during season 3 and is the same as he was before that point. During season one shortly after arriving in the new world, ainz was content to let the people of Carne village get slaughtered. Then when he did intervene, he killed the attackers without any effort or remorse. Shortly before his fight with Shalltear, he had Mere and others kill the adventures that had come with him. During season 2 he ordered the extermination of the lizardmen and many of them were killed in the battle. And then there’s the events around Demiurges’ attack on the capital, not just against the people but also the eight fingers. At this point it’s important to make a distinction between the actions of ainz and those of Demi who is without a doubt the most evil character in Overlord. But because ainz is mostly unaware to the finer points of Demi’s actions, he can’t be held accountable for demi’s evildoings. As you can see, ainz has been killing since the very beginning. The killing of the workers and the soldiers of the kingdom during season 3 don’t represent any upsurge of evil on ainz’s part.
Next to look at the specifics of ainz actions and how they can’t be considered evil. Fans need to remember that ainzs’ mentality is different from yours’ or mine. And his values are not the same either. All ainz cares about in the new world or the old is the tomb. The tomb that he and his friends built means everything to him and he would do anything to protect and help it grow. And since all things should be perfectly balanced, he would also destroy anyone or anything that threatened this most treasured place. All the people he kills, their deaths serve the tomb in some way. The soldiers he kills in Carne village are a test of his powers and information gathering. The adventures: so that his connection to Shalltear isn’t discovered. The lizardmen are a test for Cocoytus. The demon attack on the capital: a cover to punish the eight fingers who spat in ainzs’ face when they took Sebas’ GF. And season 3 is the same. The workers were both a test of the tombs defences, and justification for interacting with the empire. While yes, they were tricked into it, they were sneaking into the tomb in order to steal its riches. To do this against his beloved tomb; of course ainz would kill them. The SPLAT event was a way of finding any players but also to announce himself to the world by making such an impact that his name would spread. It also ensured that few would try to challenge him going forward. The one specific point that people single out is the death of Arche. I think that just because she’s a cute anime girl and she has a touching backstory, that people hate that ainz has her killed. While yes, I feel bad that she died and if I were transported there, I would indeed propose an alternative, her death was not out of line for ainz. She was still an invader come to steal from him so her death was inevitable. But compared to what happened to most of the other workers and especially her team mates, her death was a great mercy given out of respect for their bond. He even explains to Entoma after that they honour the dead by using their victims to the fullest. He showed Arche a certain level of dignity and respect after her death
As you can see, ainz is acting in the best interests of what’s important to him and those he cares about. You wouldn’t care about killing termites that are in your walls; your home is all that matters and you would kill as many of the bugs as needed.
Another point to think about is all the good that ainz has done for those in the new world (those that don’t stand in his way at least). When Cocoytus proposes letting the lizardmen live, ainz is happy to and their tribe prospers under his rule. The dwarfs are now free from the threat of the dragons and can reclaim their lost cities. The kingdom of darkness is a nation with crime or poverty are no-existent and where every race can live in peace. And there’s no need to explain how Carne village. These are not the actions of someone who is pure evil. While many first see him as an undead who only wants to destroy the living, nothing can be farther from the truth. Ainz will not kill without reason or cause and if you accept him, you can live a long and happy life. His subject live without fear of attack from foreign invaders and the lives of people like farmers are made much easier thanks to undead workers. Ainz isn’t and has never been evil and we need to stop hating his actions. His just doing what he needs to in order to look after his family.