@katanon
First off in a life or death situation one never attempts to connect any dots, they just go and solve the problem. Second off, there's no evidence that this isn't a commonplace occurrence in his first life post his village invasion. There might not be any proof either, but an authors full backstory is typically never written because otherwise it'd be a book and not a manga.
Order of events: I survived up to- oh i'm dead. But now i'm back! And I have years of experience on the battlefield. Maybe I can change things
A year of thinking later: Well, shit. I have no political power around here, and anything I say would be put off as the ramblings of a madman if I try too hard. And even if I demonstrate magic powers that I somehow learned myself, at the age of 5 no less, to everyone, what's the chances that they kill me thinking i'm the demon i'm talking about and i'm tricking everyone? Not high, but not low. Thus I only use magic to those I trust. The key thing everyone tries to ignore whenever these 'share your magic damn it' situations comes up is that, magic is fucking magic. Magic is magic because not everyone can use it and it's not developed to the point where it's technology. So if a 5 y/o came up to and said "Daddy, i learned how to shoot fireballs out of my fingers and I destroyed the tree over there!" right now we might go "oh! nice! magic!" but in a medieval setting? like wtf?
Lastly, the information he had to work with is monsters came in and killed everyone. Thus, solve that problem. You're right, there was evidence that demons can and do know how to control monsters. But that's not a 1 - 1 connection if on the condition that it was in fact commonplace after that first invasion and the hell he lived in. And do we know any of these things? nope.
