There are lots of reactions to have, and confirmation bias will shape many. We can blame our gun culture, we can blame our rape culture, we can blame the poor mental health care system, and we would be right....
I just one to pick on one thing. We should immediately and clearly reject the label the killer* gave to his video. This is not about retribution. Why? Because retribution tends to be defined as a just, moral, proportional response to an injustice or crime:
2: the dispensing or receiving of reward or punishment especially in the hereafter
Forget about the just/moral/proportional part. Focus on the injustice or crime. The killer seemed to think that girls and women owed him attraction, affection, love, sex, whatever. His rage was aimed at females because he felt that they had wronged him by not giving him that to which he was entitled. But since no individual female nor the available pool of females in the killer's life has a responsibility, a debt to service this killer's needs (or anyone's), there can be no retribution. There is no crime or injustice that one must punish.
Sure, it can be frustrating to see those that you are attracted to choose someone else. I was incredibly frustrated in my teen years due to my failure to attract the opposite sex. It was crushing to my self-esteem. But that is what happens one when reeks of desperation. It was not a musk I wore well or that anyone does. That this guy had no success with females clearly shows that the females in his life demonstrated excellent judgment. I am sure they could sense the desperation, the neediness, the narcissism (as a narcissist, I resent him making the rest of us look so bad).
I am glad to see that today's CNN story got scrubbed a bit last night, as it used his preferred word, retribution to describe his actions. No, it is the label of his video, and he surely imagined he was engaged in restribution. But this was not retribution. It was madness, it was rage, it was awful, but it was not punishment, it was not compensation. I don't know to react to this except to deny the killer what he wanted. I am deliberately not naming the killer since I want to deny him fame and agency and his preferred label for his crimes. Which is what he wanted as the extreme narcissist the demonstrated in the manifest that I did not read myself.*
*Yes, the old "something I read but not read myself").