The fact that the two children on the cover are covered in blood is almost fair warning -- as much as they could put on a picture everyone will see. This is one of the most graphic, grisly comics I've ever seen. I had to ask a friend if I'd been out of the comic scene so long that this was now the norm. He assured me it wasn't.
The extensive gore is the point of this narration. Kick-Ass is a young man who wants to be a superhero. This story is trying very hard to show what that would mean in real life. It means that real fights are messy and repulsive, and that people who die in those fights don't do so neatly. It means that courage and determination mean very little against superior fire power. It means that if you've never fought before and you decide to jump into the ring without training or backup, you're going to get the crap beaten out of you. It means that violence is, well, violent.
It means that many comic books have been lying to us by glossing over this last point. That's what makes them so entertaining. And it's what makes reading Kick-Ass
One more point. I don't know how she's portrayed in the movie, but make no mistake: There is nothing cute about Hit Girl. Her upbringing has been so saturated with violence that she smiles sweetly as she crushes a man to death.
Well, we know she's a freak as soon as we find out that she's being -- yep! -- homeschooled. By a father who "tells her everything she needs to know": extensive details about weaponry, what to do when a junkie pulls a forty-five, and the dictionary definition of a Democrat ("a ****ed-up prick who will march for the right to murder babies, but hold candlelight vigils for serial killers"). Yes, I understand why how her father raised her was important to the plot; but I'm never thrilled to find yet another story with the message that when you need the ultimate freak, find a homeschooler.
Anyway. This story says what it needs to say extremely effectively. What it needs to say is quite disturbing. Don't pick this up lightly, and do NOT give it to your kids without reading it first.