You can call it extreme plot convenience that there just happens to be a human enemy army marching in to invade when Rimuru needs thousands of human sacrifices. Of course it's all related since the deaths Rimuru needs to deal with were caused by the preliminary strike by that human side, but nonetheless the whole moral side is pretty much reduced to zero. You are doing nothing wrong if you defeat an invading enemy army. In fact, you are doing things wrong if you don't defeat it, from a leadership point of view. Rimuru is responsible for the nation and people he's overseeing, so it's his moral obligation to defend them.
Imagine the same situation if there was no invading army and Rimuru would need to select a random town in a human country to annihilate, for the sake of becoming a demon lord. That would make him like Ains from Overlord.