Let your guard down? No worries, it happens.
You already have the ultimate hidden skill "Isekai Protagonist" so I'm sure it'll work out just fine in the end.
"once again I have cut another worthless object"
lupin reference?
I have no idea if it was there in the original or if it was added by the scanlators xD
If you don't mind spoilers and some clues to some other things that won't be revealed for quite a while, if you look at the silhouette of the person in the last panel, you can see the "horns". They're from someone you've seen before.
https://mangadex.org/chapter/415934/2 And in Chapter 29, you see someone who has those "horns" and who has a Spider Theme in her War Room.
You can notice a bunch of powers that the Spider MC has used in previous chapters.
Based on this, you can easily make the inference, that the current Maou is a really really Old and Powerful Spider monster evolution. And sadly, I have to spoiler this next part because that's her motivation for getting involved.
Who doesn't take kindly to her descendants being murdered.
There are a ton of other things actually clued in that chapter and scene that I'll avoid spoiling.
Until now we had seen 2 points of time line.
You need to remember that monsters aren't humans and they are ageing in different pace.
What we had seen until now are 'Kumoko Timeline' where she slays dragons while her classmates who "got lucky and reincarnated as humans" are still crawling as babies in most of cases (few of them became non humans like her)
And we had seen Demon Council (flash forward) showing us bit of future and few important characters, where class members should be teens if i remember right (been some time since i read this novel)