If this was a kid who had gotten screwed over by bullies for a long time -- and not just "he called me fat," but ones that were genuinely threatening him with physical violence, I can completely understand why he would carry a weapon. The problem with that is, though, that you can't point a weapon at someone and not use it. It's like if you have a gun -- for one, you should respect that weapon enough to keep it stowed. If you point it at someone, you must be willing to use it. For another, the moment that weapon is out, either you have to use it, or the other guy is going to take it, and suddenly you're incredibly screwed. Even if the guy was just going to mess you up before, he might be so pissed off or unthinking that he kills you with what you had for defense.
For those who say "11 times is a lot," I echo everyone else who says it isn't. But we've heard the "arms move fast" argument -- here's the other. Adrenaline makes you not think. The idea is that if a big predator is coming, and you have to fight, you're going to do it based on gross motor skills and instincts. Average Joe has no ability to really control what he's doing once the fight is on. He can choose to not fight (not an option for this kid, clearly, because he tried and failed), he can choose to curl up in a ball once the fight starts (a terrible plan if someone is clearly intent on threatening your life), or you can do Something. What Something is, is determined mostly by your animal instincts and your subconscious, because you're going to do whatever comes to muscle memory first. Depending on who you are, it might be a punch; it might be picking up a big stick and swinging; it might be martial arts. If you have a knife, and it's in your hand, you're going to use it, and you're not going to stop using it until the threat is gone, and odds are you're not going to even remember what you really did in that span of time. People have been shot or stabbed and end up blocks away from where the fight happened, only to suddenly realize consciously that they were running away at top speed and have been going for a while and, oh wait, I'm bleeding all over myself from a big knife wound in my leg, this kind of sucks, maybe I should call 911 before I fall over and die on the sidewalk.
Also, being stabbed, while it sucks a lot, is not going to stop someone unless it's to somewhere major. Sure, he stabbed the guy in the gut, but it was a pocketknife, those are fairly small, and the angles might not have been deep. If you just cut a guy's muscle, unless you sever something mostly or completely, the guy is just going to get pissed. If you take out an artery or something, then it gets serious. As I was always taught, you fight until the guy is done. The point at which he's no threat is the point at which you quit. This doesn't usually mean "dead" -- most of the time it means "unconscious" or "in too much pain to fight" or "fleeing" or something. But if the only option for this kid was to keep going, if they were grappling close or something, then it might have been that at some point, stabbing him 12 times was what it really took to win.
In short, the kid did exactly what nature told him to, and he had a lot of odds stacked against him, and once he was in the fight (that he didn't start), what he did in his defensive panic might not even have been under his conscious control.
If a guy wants your money, you give him your money. If a guy wants your life, you take him out. I can't say I give the kid props for actually killing the guy, but I do give him props for defending himself, and I also don't think he should be tried as a murderer. Manslaughter, okay, I can see that. And I certainly think he should be encouraged (or even instructed) to receive psychiatric help for his issues related to the bullying, and also to the fact that *he just killed a person.* But he didn't just walk in there trying to pick a fight so he could kill a guy.