When James Gunn re-introduced Peacemaker, aka Christopher Smith, in his own wacky way in The Suicide Squad, no one knew that the character would blow up into being one of the more recognizable DC characters today. Portrayed by John Cena, the character has had a resurgence, and a movie and a show later, he is quite popular now.
Before The Suicide Squad and the Peacemaker series, the character wasn't well known outside the comic book community. He was somewhat obscure, which gave Gunn a blank page to shape the character's personality however he wanted. Hence, whenever you read an old comic with the character in it, you will understand why he acts so differently.
Peacemaker's comic book counterpart is quite different from his live-action one
Believe it or not, Peacemaker never began as a DC Comics character. He was originally introduced in Charlton Comics' Fightin' 5 #40 in November 1966, created by Joe Gill and Pat Boyette, and was bought by DC Comics in the 1980s when Charlton went under.
Christopher Smith, aka Peacemaker, is quite different in the Charlton comic books. He is a diplomat committed to peace who would use his superhero persona to fight crime. He had various non-lethal weapons at his disposal and was pretty much a pacifist who would take on dictators and warlords.
However, under DC Comics, he was changed to be a veteran of the Vietnam war who was quite mentally unstable and was a war criminal. He began working for the government as he believed he received a second chance and took up the alias of Peacemaker.
Smith is also the son of Wolfgang Schmidt, a Nazi Death Camp commander. When he saw his father shoot himself in the head, he began believing that his father's ghost would haunt him. He also believed that the spirits of those he killed were trapped under his helmet as a part of his mental illness.
In the live-action, Smith is a very different character. Being the son of Auggie Smith, a racist supervillain famously known as the White Dragon, Christopher had a pretty ugly childhood in which he accidentally killed his brother and was trained by his father.
A tonally different character, this version of Christopher Smith believed that he would do anything in the name of peace, no matter how many men, women, and children he would have to kill.
It's overall more comedic and sort of fits under James Gunn's vision. He is also a part of The Suicide Squad, which the original Christopher Smith from the comics was never a part of.
In some instances, Gunn's version can also be considered a straight-up supervillain or someone who has at least retained a very anti-heroic persona. But whatever it may be, we are still glad we got it because John Cena is extremely entertaining in the role. Here is hoping we get to see Peacemaker again soon.