Well, it’s technically always about the killer. But this one is unique in that he never should have gotten that far. Not even close. That’s why he is almost a perepheral story here. Sure, he needs to be hung, electrocuted, shot, or whatever, but the real story is his getting past several layers of keystone cops.
To use an analogy: if a shooter assassinates the president, he needs to be executed, but the real story is how the heck did he get so close to the president?
I certainly don't want to fan any conspiracy theories, but at this point in time, other than the reported confession of Cruz, what evidence exists that he actually was doing the shooting? Has any news media shown an interview with a student who ID'd Cruz? There should be plenty of students and teachers who saw him shooting.
One teacher reported that the shooter was masked, and in black, which would make identifying the shooter difficult for witnesses on the scene.
I'm not suggesting that Cruz isn't likely to be the perpetrator, but I would think that there would be more reporting of eyewitness accounts.