I try to keep things simple, which means looking to the past.
Let's take the Seven Deadly Sins: wrath, avarice, sloth, pride, lust, envy, and gluttony. The worst one is Pride, or what the Greeks called Hubris, because it is the basis of all the rest.
If you look at the story of the Garden of Eden, it started with them wanting to be Gods (Hubris). Then there is the serpent (a symbol of envy and therefore hate)...and Adam and Eve listen to him and get kicked out. The lesson? Pride is the worst sin, followed by envy. And the Seven Deadly Sins is what brings all human evil into this world.
Rodger was afflicted with hubris (which the Greeks considered a type of insanity if bad enough), calling himself "the Supreme Alpha" and a God and other hubristic things. And he was eaten alive with envy. And he was wrathful, which doesn't mean angry. It means wanting to kill others, and sometimes yourself.
So here we had a man who suffered from Hubris, was consumed with envy, and who was wrathful. And there was lust. What about avarice, spending his money on the lottery, hoping to become wealthy?
And that is why he did want he did. Not because of lack of gun control, or misogyny, or Pick-Up Artists, or whatever nonsense the Stupid/Ignorant can conjure.