Which is why it’s bad writing.
“Unblockable killing spell” is the kind of thing that pops up on a middle school playground because every kid wants to have the trump card in make-believe and the last kid just cast Meteor.
Eragon is a contemporary-ish book and has killing magic that can kill normies by the dozens/hundreds, but other magic users have to do more than play rock-paper-nuclear-option.





Yes, IIRC the “Unforgivable Curses” either carry a death penalty or wizard prison forever or something. But by the third or forth book every bad guy is a Wizard Nazi who doesn’t care about those consequences and the good guys are all fighting Wizard Nazis and are at least semi-justified in trying to kill them right back.
It basically devolves instantly into gun-fights, except instead of guns it’s a specific spell.