Weary is a sad bully who attempts to beat up Billy Pilgrim while they are both on the wrong side of enemy lines. His nasty obsession with torture devices marks how deluded he is about the realities of war. He thinks all of these instruments of pain and death (like his three-cornered knife or the Iron Maiden of Nuremberg are neat tricks. But when he is actually on a battlefield, all he can do is fantasize about what kinds of heroic stories he'll tell his family when he gets back from the war. His blindness to the realities of the violence around him leaves him exposed to capture by the Germans. Weary's fantasy life is so extreme that he imagines that it is Billy who is responsible for his eventual death by gangrene. Even as he is dying, he can't let go of his stupid, petty resentments.
Paul Lazzaro spends the book in the middle of the most awful situation a soldier can find himself in: as a prisoner of war at a time when there is no food and nowhere to house enemy soldiers. And Paul Lazzaro, like Roland Weary before him, takes this awful situation and makes it ten times worse for his fellow captives. He constantly threatens violence, promising everyone he has a grudge against that he is going to have him killed after the war. He dreams of revenge and raping women. Even though Lazzaro has no power to carry out any of these terrible threats, the fact that he can sit in a war zone and want to see more death and more suffering is a mark of how crazy people can get. It makes sense that, out of all of the POWs in the novel, it would be Lazzaro who promises to kill Billy in revenge for Weary's death. Both Lazzaro and Weary represent the kind of bullying, stupid guys that war seems to bring out of the woodwork.
Bertram Copeland Rumfoord is the opposite of Billy Pilgrim in every way: he's this incredibly healthy and athletic 70-year-old who was born rich and is married to a hot, young wife. Rumfoord looks at Billy and sees a pathetic weakling who deserves to die. If Weary and Lazzaro are the kind of guys who make wartime violence so bad on the battlefield, Rumfoord is kind who makes this violence possible from off the battlefield.
Rumfoord believes that might makes right, that stronger armies should make their enemies suffer, and that Dresden was an incredibly successful air raid that should be celebrated. His planned history of the Air Force, in which he wants to glorify the Dresden firebombing, is the kind of book that makes war seem reasonable and positive. Guys like Rumfoord help popularize and glorify war– and in an anti-war book, that makes him an absolute villain.