As I recall, he was a seriously mentally disturbed. He never should have been allowed any access whatsoever to firearms. The weak link in the chain was his own Mother. She enjoyed shooting sports AND was overly trusting of her unhinged son. She both taught him how to shoot and, if I remember correctly, allowed him ready access to her own guns.
Need to reopen all the mental hospitals closed in the 1970s. Only three mass murders before then, one Howard Barton Unruh in New Jersey, with a Luger pistol, Richard Speck in Chicago with a knife, and Charles Whitman in Texas, with a bolt action hunting rifle. And none used a semi-auto rifle which had been on the markets since 1903.
After the crazies were released on the streets the mass murder rate took off like a rocket. Almost all would have been in a mental hospital if the mental standards of the 1950s were still enforced.