That Time I Got Reincarnated as a Slime: Tensei shitara Slime datta Ken

[KrayZ33]

The only thing you can negotiate with an invading army is your own surrender. Rimuru would have needed to slaughter thousands of soldiers no matter what to convince the rest of fighting being futile. However, like DE said, if given a proper chance, the army could have possibly offered real resistance, including being able to hurt Rimuru.

Where is that comming from though? If we go by what we've seen and with what the characters in this show knew and said so themselves, then there was no real threat.
There are probably going to be deaths depending on how it turns out and how fanatical these soldiers are.
But this was just him farming souls under the pretense that "no mercy is necessary" - which he decided on his own just like that and honestly, I doubt he ever really thought about it in the first place.

on a side note:
I kinda also wonder what the barriers created by the holy-church faction actually did in the end because it didn't seem to affect anyone all of a sudden.