I do have a feeling Rimuru wanting to kill all those people on his own wasn't only to be sure he's going to collect the souls but also so that none of his underlings would need to bathe in blood. His wish for a peaceful coexistence was, for sure, genuine, so what he's doing now is not something he will be proud of.
War is evil business. It's all about maximum death and destruction. The only sliver of justice in that affair is on the side of the one who didn't attack but defended. However, it changes nothing about the need to kill. Ironically the attacker is the only one who gets to make a choice, the defender is forced into it.