I have a hypothesis given the confusing nature of this case. The killer had 47 guns at the time of death but was not known to be a collector of guns before then. His relatives are confused about him having any grievances and mental issues prior to the shootings. The killer mounted his automatic gun on a tripod. He seems to be going overboard to buy and use as much deadly military hardware that the law allows. I think this is the key to understanding his motive. His actions don't seem to make sense if you think of it as a typical mass shooting. But his actions do seem to make sense if you think of it as the killer making a very deadly statement about laxed gun ownership laws in America. He's saying:
"Look what I - anyone - can do because of the foolish, laxed laws about the right to bear arms! Look at how much assault weapons one citizen can buy while you idiots worship your constitutional right to bear arms! Look at how much people can be killed!! When will you people wake up?! Don't wake up, just keep sleeping and partying out in the open while your laws allow me to easily do this..."
That's what I believe this was. This was a guy who was against the very laxed gun ownership laws and went off the deep end to make a deadly, extreme statement about it by carrying out an elaborate mass shooting with an overkill of guns that was completely unnecessary.