What kind of man is Billy Pilgrim in "Slaughterhouse-Five"? Why does Kurt Vonnegut choose to make a man like Pilgrim the protagonist of his novel?

The protagonist of Vonnegut's "Slaughterhouse-Five", Billy Pilgrim, is a quintessential anti-hero because he is an unequivocal vehicle for anti-war sentiment, yet lacks any admirable or heroic qualities. An optometry student before getting drafted in the military in World War II, Billy is nerdy and awkward. While many of his peers display the bravado and assertiveness expected of a soldier, Billy is ridiculed for his physical and mental fragility; he has frequent mental meltdowns and is consistently passive in the way he experiences everything. A case in point: he and his fellow POWs survive one of the deadliest massacres, the firebombing of Dresden, by hiding in a meat-locker. The irony that he survives the war without contributing any dashing or heroic effort underscores the futility of war and the arbitrary, senseless loss of life. 

Vonnegut's decision to characterize Billy as a pathetic weakling is essential to deconstructing the generic archetype of the hyper-masculine soldier. Vonnegut wants to make a larger point about the depravity and inhumanity of war in spite of its frequent glorification in literature and history. The best way to achieve that is through a sensitive character like Billy, whose military experiences leave him mentally scarred, begging the question: is surviving war more desirable than dying? Through Billy's PTSD (Post Traumatic Stress Disorder) and deluded belief that he has been abducted by aliens, Vonnegut indicates that war has taken a huge toll on Billy's mental well-being. Thus, Vonnegut successfully uses Billy as a tool to critique the destructiveness and pointlessness of war.