Rejection is one of the most difficult emotions to deal with and process. Men need to be able to feel they can express themselves and their emotions. Going to a counselor isn't the answer either- so much of the problem is people who should love and care for their own pawning them off on professionals because frankly they don't want to face the problems themselves. Our society (U.S.) has become a society of avoiders and pill takers. We need for familys and friends to step up and help with each others internal pain. Who knows what this kids background was like but at the root of his motivation was he couldn't handle rejection.
For the person on the receiving in end it does feel like being treated like property, the entitlement you speak of. But I think (an opinion so obviously the lowest form of thought here) the rage is born out of not having a proper emotional "valve" to release.
I mean, speaking from my own experience, growing up was a shit show in terms of learning to express myself. It wasn't until I was an adult that I realized how much I put my anger on other people, or back at myself. The issue is that men are socialized to be like this because of anachronistic and toxic ideas about what 'men' are supposed to be like.
I think a counselor is great but I agree that the U.S. in a larger sense suffers from a kind of country wide horror of alienation.
My main argument in the post above was that this shooter fit an archetype of an entitled, resentful, alienated (white?) male. And the basis for their identity isn't natural, but socially developed. I agree there is no valve for release, but take it further. Why is the rage there in the first place?