I agree, but you have to know that he also didn't want to start a war. In the anime, he explained that he doesn't want to hurt humans because once they're hurt, the humans will bring their friends to get revenge, this will get bigger and bigger which can led to bigger casualties on the monster side. Yes, I know the monsters are strong, but the humans are also strong here, and wars can lead to a lot of casualties on both sides. Rimuru basically wants to avoid this too. But yeah, I also think that Rimuru should revise that rule, like put so condition on it or something, cause the monster took it literally
*some condition
and he did. He revised the rule that their actions are solely based on how humans act towards them. If they are friendly, they will be friendly and if they aren't, they will retaliate.
Honestly rimuru navie stupidity caused this, don't kill humans??? Just because they are monsters??? Humans kill themselves so it doesn't make any difference. As a leader, he shouldn't have been relaxed since he was human, he knows exactly how humans are. People lost their lives because they were trying to honour his wish of not killing humans. Thats why the casualties are soo high