There is no one reason for the recent killing spree in Isla Vista.
There are a myriad of reasons. Although I'm sure mental health played a part, plenty of mentally ill people don't kill other people. But what strikes me as unique to this particular crime, is the way in which his rampage was fueled by misogyny, as many feminists have pointed out.
I won't rehash their arguments, but I will reflect on discussion of his mental health as a way to minimize his ideology.
Looking at these videos and the very common theme, which is that Rogers feels sex is owed to him, and that women are unfairly denying that to him, one can't say that misogyny didn't play a role.
The question is "How much of a role did misogyny play?"
In my opinion, it played a pretty strong one. It would be easier to simply leave it at "mental illness" even though we don't have the facts on what his illness was, because somehow that makes us feel comfortable. He was an outlier. Despite the fact that there are many mentally ill people in the world, we have been taught that "crazy" people are rare. If Elliot Rodger is simply one deranged killer, we don't have to look inside ourselves and examine our own misogyny. It is scary to think that this is lurking inside so many of us, and so many people we know. I have spoken to numerous people, whether acquaintances, co-workers, college classmates, high school classmates, etc., who have expressed themselves in a very similar way to Roger.
That is why the first video terrified me. I certainly did not watch it to the end.
What's worse are the YouTube comments on his channel, many of which pondered whether or not this all could have been avoided given those "blond sluts" Rodger referred to had just offered the sex he obviously deserved. Then all of this mayhem could have been avoided, they argued. That sort of commentary should serve as proof of how widespread this attitude really is.
Although (for all we know at this point) his mental illness could have driven him to punish women with such severity, it certainly isn't the main reason he decided to exact violent revenge on women. If he didn't kill all of these people, it is quite possible he would have committed another violent act, assuming he were perfectly well. To not examine the ideology behind it is to dismiss other ways men exact revenge on women for the lack of interest every day. Less severe, but impactful acts of violence occur every day. Mental illness does not create the drive to kill women for very specific reasons - reasons that you can find on many, many websites.
They're too specific, too familiar, to be discounted and pushed aside as the ramblings of an unwell man.
For better insight and commentary than I can provide, please read here and here and here.