Legal or not, they were effective. The maggot planned his moves, armed himself with what he determined to be the right weapons and went to town. Dealing with the how is easier on the mind than dealing with the carnage.
Now the public is desperate to know the why, which may never be truly known. Whatever motivated him, he saw it as logical in his demented mind. Sane people will never accept such, they need a logical reason for an "illogical" act. It's the old "how could and why would someone dismember a body?" An answer, "So it would fit in the bag/box." Sane people have trouble understanding, they need/crave clear cut, easily understood answers which are not always forthcoming.
Some are just fascinated with the tools, others on the inhumanity and tragedy of the act. "First responders" focus on the job at hand, shutting out the carnage in order to function. The confronting of the carnage necessarily comes later.
The shooter could have accomplished more if he's loaded his plane with fuel and crashed it into the venue. I think he wanted the gratification of prolonging and enjoying the carnage. Or, perhaps he didn't want to defile hi personal enjoyment of his plane. He didn't want to simply kill a bunch of people, he wanted to see witness the spectacle. I make that observation based only on some experiences, knowing it is an exercise in futility.