I think it’s the sheer enormity and brazenness of the Saudi actions.
In other circumstances, Turkey might have approved of the assassination of a liberal, independent reporter, and the collateral chilling effect on liberal, independent reporting. Erdogan certainly knows how annoying they can be.
But this one was even worse than Russia, which is saying something. They should’ve gone the route of Israel, which at least tries to commit its murders in a deniable way.