In the USA, if he had been involuntarily committed to a mental institution, a felon, dishonorably discharged from the military, had any time of domestic abuse charge, PFA against him, open warrants for his arrest, history of drug use, then he would immediately by flagged by the insta-check system that ALL ATF licensed FFL dealers use when selling firearms. Either that, or he obtained the firearm illegally by stealing it, or buying it from someone else on the street who stole it themselves. That, or he obtained it legally because he had no priors (like I mentioned above), and then went nuts.
Crazy is crazy. If he wanted to kill them, he could have doused them with gasoline and lit a match. He could have run them over with a truck as they walked to their newsvan. He could have dropped cement blocks on their heads from above as they left this interview. If someone is determined to kill, they WILL. Look at all the knife/machette attacks in China, Asia, etc. Sheesh.