Sure, it's different.
The thing is, Rimuru is making an example, he could probably have solved the whole thing in a different way but he doesn't want to, nor does he actually care.
That's the deal about what makes him evil in this situation and morally liable.
"It can't be helped I guess" *proceeds to slaughter the army of 20k in seconds*
No, he is wrong, it can be helped. It's just easier to get what he wants with this.
You might even say that he is happy about it. In the end, he gets a chance to revive his people this way. He doesn't care about the fact that he basically lost all the "lets avoid fighting"-reasoning he had before this event.
The army marching on his nation is obviously not a threat when he can just delete it like that.
It's a joke.
In our world this is the equivalent of some nation sending a single soldier to declare war and the war is lost for the attacking nation as soon as that single soldier has been shot.
Is it that nations own fault? Hell yeah. Could Rimuru have done things differently? Fucking hell yeah too. I just don't want this to be viewed as an act of last resort or some bullshit like that, because it truely wasn't and isn't.
He could've found different ways, and he could've stopped that army in a different way and probably didn't have to kill them. He is basically a god and he tries to make it clear that no one should fuck with him or his nation by killing thousands. That's not benevolent - that's all.