I can see where you’re coming from but it says shinichiro is from another world and he has blonde hair while the king has black hair and from that world. My guess is that the king was the former hero but he secretly loved the Demon Queen ( who I Stan). Since the Demon Queen was married to Shinichiro, he took Shin. out the equation for 2 reasons: 1. Capture the Demon Queen for himself and 2. Stop her from having more children (with the man who’s not him) hence preventing the prophecy killing two birds one stone. I think his possessiveness stems from his pride/duty/jealousy/rejection/anger/ love-hate feelings towards the Demon Queen!
I've been thinking about this for a while now, but me thinks the Human King is Shinchiro, like I always feel his possessiveness/desire to keep Dernière locked up was more than him like wanting to keep people "safe" but Idk