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.
This seems equivalent to someone buying a hammer from a pawn shop. Upon authorities finding out that hammer had been stolen or used in a crime instead of charging or investigating the Pawn Shop owner, they charge the person who bought the hammer because they made things with the assistance of "stolen goods".
Used car / firearm / AI-assisted-code / other-example-here