Having now watched the anime adaption of this scene as well as reading the manga and ln versions (not the wn), I can say I greatly prefer the manga's take. The anime doesn't give Rimuru's decision proper weight, it's too fast and too subtle. In the ln he immediately thinks "Oh I only need to slaughter 10 000 humans to bring them back? Deal!" (note: this is before he learns of the approaching army) which is a bit too callous in my mind. In the manga he thinks it over, what he did wrong, how naive he was, braces himself, and then decides too do this great moral wrong for his own selfish desires (this is again, before he learns of the army). And when he finally hears of Farmus army he is glad and cold.
It's a fantastic moment. Which the anime robs all power from by not letting Rimuru make his decision before he learns of Farmus Army. The moment is supposed to show just how far Rimuru is willing to go, even butcher innocents, if he has too to bring back those he loves. Which importantly is a selfish desire. We tend too think of Rimuru as altruistic and yes he is a pretty nice guy, but if you look at what he does it also always falls in line with his personal desires. This is a guy who created a whole nation because A: he couldn't say no to puppy dog eyes and B: he wanted to live an easier life of comfort.
Rimuru's duel character of selfishly altruistic is what makes him so interesting. And is why this moment where his selfishness trumps altruism is so important.
