What drags this “Squad” down to the dreary level of Ayer’s vision is the tone of Gunn’s film, which is more violent and less lighthearted than his “Guardians” movies. Despite needle-drops from Johnny Cash and Louis Prima, among others, the soundtrack doesn’t play like the mix tape that helped make that franchise so entertaining. And while the squad cracks jokes, the humor is dark and bloody, just like his cruel charges.