You might have noticed that there is a lot of blood in this novel. Spouts of blood, sprays of blood, gouts of blood. Buckets of blood. So, what's with all the gore?
Most obvious, of course, are Jack's bloody noses. Whenever Jack is stressed out, upset, or scared, which is a lot of the time, blood shoots out of his nose: "I noticed a bubbling river of blood running out my nose and across my lips" (7.2). It's pretty clear what Jack's nosebleeds mean: he has little control over his emotions and his reactions to events. When his nose stops bleeding at the end of the book, it's a sign that he's starting to grow up.
Sometimes, blood seems to represent emotion more broadly. Take the moment when Jack's nose leaves a "big red splotch on [his] shirt [that] looked like a real bleeding heart" (9.114). You've heard the phrase, "bleeding heart liberal," right? This means that someone is excessively compassionate and cares deeply for others' welfare (perhaps to the point of being naïve). Think Jack's mom, who cares for the poor so much that she grows her own corn.
Well, that's pretty much what this image means. Jack cares. A lot. Plus, it also gives us a neat contrast with the Hells Angel's tattoo of a black heart, which we see just before Jack's "bleeding heart." Black heart? Cares nothing for others. Bleeding heart? Can't stop caring.
But blood works hard as a symbol. When Jack buys the poison for Mr. Spizz, his nose bleeds a little more ominously. As he signs his name on the poison purchasing form, the bleeding starts up: "Just then a drop of blood slid down my lip and plopped onto the soft paper where a moment before I had written my name. I stared down at the red spot of blood as it spread through the paper fibers. I didn't like how that looked" (15.39).
Good call, Jackie boy. For Jack, bleeding on his own name as he signed the purchasing form is a "bad omen" (15.42). It foreshadows the murders, and since we later find out that buying the poison basically makes Jack an unwitting accessory, it's a bad omen, indeed.