Lord of the memes

I wouldn't say it doesn't make sense. Movie Denethor looks like someone who has completed and utterly cracked under stress.

Not only did his favoured son die, but he died after not wanting to go to Rivendell and Denethor forcing him to go and take the ring. Denethor blames himself for the death of Boromir.

Add to that the fear and pressure of bordering Mordor at a time when he knows Sauron is preparing for war and your city is right on their doorstep, it doesn't seem that crazy that he cracked.

He certainly has more depth in the book, and there's more sympathy towards him, but he still makes sense in the films IMO.