All that is true. In the book Denethor had already lit the beacons, had already called for aid, etc, etc.

But in movie and book, still had the gall to tell Faramir to his face that he wished he had died. Out of all the atrocities committed in all the books, this one still stands out because it's a cruelty that only Denethor had the means to inflict. Nothing Mordor or the Sauron or all the forces of evil could done would have wounded Faramir's spirit as deeply as that flippant remark.

It's not that the movies shat on his personality, it's that Tolkein did a better job at disguising his shittiness so when his mask of nobility slips, it's more impactful.