What a fucking loser. I swear greedy ass men ruin everything for everyone cuz why was this...

Considering succession battles usually end in assassinations and that he likely went through one of these in his youth, it's pretty understandable.

It's more paranoia than greed, and justified paranoia since even if ML gives up the throne, there's no guarantee that nobles won't rally behind his potential future kid. With FL's dad being powerful/wealthy and caring towards his daughter, their family has a good chance of inheriting all of his wealth in addition to the ML's power. A kid with that kind of wealth and backing, if ambitious, may even end up killing his daughter and grandkids. It wouldn't be the first time in history where a relative of the ruling family does some murder to move up in the line of succession.

And even if no one in ML and FL's family try for the throne, they'd become powerful enough that they no longer need be absolutely loyal to the crown. That can eventually devolve into nobles being more loyal to ML/FL fam than the royal family, meaning you'd end up with a puppet ruler reliant on ML's favor. Worst case, if the ruler pisses off ML's fam for any reason, he could undermine the ruler's power or even secede or usurp the throne.

If the king is wrong, he just elevated his blood pressure for no reason, but if he's right, he's looking at someone very dangerous to his dynasty. It's the same reason dictators sweat if their generals get a little too popular/powerful.

This isn't unexpected either, the king has been shown to be very shrewd from the beginning. This whole thing is a conflict setup for when FL inevitably becomes pregnant. Pretty sure FL and ML will suddenly lose all apprehension towards having a kid when that happens. Ig he doesn't trust his daughter to take care of herself since she fully supports ML and FL. Her husband's background not being as powerful as FL also possibly fuels his paranoia.