I disagree with those saying that the MC is dumb for letting the 3 idiots live.
But he is definitely braindead stupid.
They already suspect him because of the princess behaving "out of character" and his great idea is to scare them with overwhelming strength and terrifying skills.
Objectively speaking, they are pretty much justified in their reaction. Not 100%, but pretty close.
The only thing they should have realized, which is easier said than done when you're in full panic mode, is that they are still alive. Which likely wouldn't be the case if MC was really a demon lord.
All in all, I blame MC for 80% of the idiocy in this chapter, the princess for 10%, and the hero's follower trio with the remaining 10%.