I disagree a bit and would like to apply some perspective. Killing other human beings is always wrong, that's besides the question.
I don't know what this guy's motivation was, but for many, being bullied in school is a huge contribuying factor. There's really little worse in the western world than bullying. I'm now past 30, but I still remember the bullying from school.
These guys would wait until I was alone, then hurl insults at me, hitting me against my shoulders repeatedly, all the while grinning with the highest degree of satisfaction. But you cannot do anything against it. Eventually I fought back, but guess who got punished. Mhm.
Years later, these bullies now have a successful career, and people praise them for changing from the jerks they were to the upstanding citizens they're now. Meanwhile I go to therapy, have no career to show for, and struggle with both obesity and depression.
That's not fair.
Had I been living in the USA, where I can grab a gun easily, I don't know what I might have done.
And that's not a unique story at all. But adults don't give a crap about bullying. Teachers are afraid of parents. Parents never accept that THEIR sweet little angel did something so terrible. And on it goes. The only thing you can do when you're bullied is to tell yourself 'only a couple more years, only a couple more years'. Or grab a gun. Fortunately, the latter isn't that easy in Europe.