So at the beginning I thought he was like that because he lived all his life as an assassin. Without experiencing a normal day once in his life. You know, because training and all that.
But the more I read, the more I'm starting to think the MC maybe mentally handicapped. A retard.
There are situations where he could have understood everything by simple logic and acted normal. If only, if only!! Man, problems could have been avoided. But he fails.
To the point that it doesn't feel like the usual japanese "MC-lacks-common-sense" trope. It feels weird.
*PLOT TWIST*: The only reason why he was trained as an assassin was because he is in fact retarded, and that made him easy to manipulate/turn into a killing machine. That's why he's so good at it.