Ol' pointy ears has been borderline nuts from the very beginning years before Frank Miller was even born. Watching his parents murder when he was a child clearly left him with psychological issues.
The Batman persona wasn't just a mere disguise for him -- it was who he considers himself to be. He thought of Bruce Wayne as a disguise.
Batman was sadistic. He clearly enjoyed beating up criminals as he smiled and laughed and wisecracked. He even went around killing -- with a gun no less -- in the original early Golden Age.
From the very beginning Batman certainly had childhood trauma, abandonment issues, neurotic obsessive compulsive disorder and anger management issues. Robin the Boy Wonder is another example -- trying to recruit children to help him in his war.
Taking a child and putting that child in a costume out in danger in the middle of the night. He would be on the rooftops looking for crime until three or four o'cock in the morning all the time. Every night of Dick's childhood was spent on the roof, even when he was too young to legally be out past curfew.
Bruce would go through his files down in a cave and then they'd go to the rooftops and walk around the roofs for hours looking at all the streets. In Batman #2 "The Joker meets the Catwoman" Batman concocted a crazy plan to abduct the Joker from the hospital and take him to a famous brain specialist for a forced brain operation.
In the Bronze Age in Batman #232 "Daughter of the Demon", reprinted in Batman Illustrated by Neal Adams volume 3 and Tales of the Demon, Batman says he has "dedicated my life to a relentless war against crime...A totally devoted, almost fanatical young man...Soon I became feared and hated...exactly as I'd hoped!"
In Detective #457 "There Is No Hope In Crime Alley", reprinted in Batman in the Seventies, Batman severely beats a thief as he flashes back in his mind to his parents murder, he only reluctantly stops because Leslie Thompkins interrupts saying "Stop..Stop! You've hit him enough!" "I lost my head!" is Batman's response.
In Detective #478 "The Coming of Clayface III", reprinted in Strange Apparitions, after Silver St. Cloud dumped him Batman takes his seithing anger out on two shoplifters -- savagely beating them as he flashes back in his mind to Silver, he only reluctantly stops because the police arrive.
Returning home he lashes out at his dead parents -- "This is all your fault! You -- my dear departed parents! If you hadn't died, there never would have been any need for the Batman! I could have lived a normal life like any other man!"
In Detective #479 "If A Man Be Made of Clay", also reprinted in Strange Apparitions, Batman meets a girl whose boyfriend was just killed by the Clayface, the girl hurls herself into Batman's arms. Tenderly Batman cradles her against him. he turns her face to his -- and sees Silver St. Cloud. Batman then talks to himself out loud -- "I've got to gain control of myself! Silver is gone! I've lost her!
And somehow I've got to accept that fact ...before I crack!" Frank Miller didn't really make Batman kookier than he had been before.
What Frank Miller did was bring Batman back to his dark roots further than anyone else had, more in line with the original vision of the character.
Closer to the police-beating, bone-breaking brutal Bill Finger-Bob Kane original. First is the use of the bat emblem on his chest without the yellow moon in Batman The Dark Knight Returns.
In the Golden Age this was the standard. Batman carrying and using guns, which hadn't been seen since 1940. Frank Miller brought back Robin's sling shot which had not been seen since 1940.
The gadgets and bat vehicles which had been toned way down in the '70s. Batman originally was wanted by the police and fought cops. Batman The Dark Knight Returns was a return to that concept.