From the information available so far, he was unlikely to have been schizophrenic. The planning of this act (which resembles that of Anders Breivik in Norway, who killed 77 people in 2011) is beyond the capacities of most people with such a disorder. Paddock’s quietness and almost reclusiveness are compatible with a paranoid personality, or with the possession of a single fixed paranoid delusion; further investigation will perhaps establish whether this was so. But even this would not explain all, for paranoid personalities and delusions are common, but mass killers are few, even if increasing in number.
Perhaps he had an animus against the city of Las Vegas that had encouraged his gambling habit, and he wanted to revenge himself upon it by discouraging its tourist trade. Maybe he was a country musician manqué who was envious of the success that he had never had. But then we should have to ask how his resentment overcame the normal ethical restrictions with which he had complied for several decades. And any explanation of this will be banal—more re-description than explanation.