The only comics I ever collected over the years were the Superman series (Action, Man of Steel, etc) and the Batman series (Detective, Batman, Dark Knight, etc). According to the Batman/Superman canon (at least in one or two story lines) Superman entrusted Batman with a kryptonite bullet in case anything ever happened to make Superman lethally dangerous to humanity.
Then again, expecting movies like this to stick to the original spirit of the characters, even ones as iconic as Batman and Superman, is probably ridiculous.
One of the stories involving the kryptonite bullet occurred in this story arc featuring probably the only DC superhero that could actually defeat Superman. The Spectre: