In reading the question and details, it sounds like the real question is why do some people think that their act of committing a murder is justified?
Serial killers can pretty much be ruled out of this equation. For those that talk about it, it more often than not has to do with self-gratification. Some even admit that they knew it was wrong and just couldn’t stop their actions until someone intervened. Then there are murders that are gang-related, self-defense, crimes of passion, accidental, and the list goes on. Each case is as different as a fingerprint.
In this specific case, all we know at this point is that a high school student allegedly stabbed a principle and allegedly gave a vague reason why. In digging a bit further into other news reports, there is no mention that this confession was given while his parents and/or an attorney were present. If anyone has ever been interrogated by a police official, they understand how scary it can be, even if one being questioned is innocent or guilty of a minor offense. Being accused of murder is huge, and a teen is unlikely to know the local or state laws.
It is good to hear that the judge has asked that the young man have a psychological evaluation. There have been far too many cases of youths being accused, making a false confession without proper support, tried and stuck in a jail when they turn out to be innocent years later because of the due diligence of someone who re-evaluates the case.
Whether this kid is guilty or not, he deserves a fair trial.