It's not like every monarchy had succession automatically to the firstborn, plenty of rulers would name their heir. Also I've played enough Crusader Kings to know how to get rid of a low-stat child!
"Good news Harold! Your older brother has decided to join the priesthood. Yes, it was of his own accord... well, maybe he needed a little convincing..."
Dudes obviously magic tho, summoning a strong enough concentrated beam of light inside with a futuristic lens design of unknown origin for the time period!
Pliny the Elder noted the use of glass vases filled with water to concentrate sunlight heat intensely enough to ignite clothing, as well as convex lenses that were used to cauterize wounds.
Ouch.
Cauterising wounds seems to have been a thing outside of movies. I'm not sure how effective it was. You removed the infectious agents but created a larger surface of attack.
Depends on where and when you are. Not every kingdom practiced primogeniture and several places and time periods had ways to remove people from the line of succession. Even when they didn't have any of that, civil wars to put a more well-liked person on the throne were a thing. Cleopatra's faction trying to crown her over her less competent brothers, for example.
Nowadays, the rules specifically select the sociopaths to put them in power.
When you put it like that, maybe divine right of kings had something going for it...