I can only imagine the feelings associated with living in a beautiful beach town as an ugly/undesirable male. Imagine being in a beautiful environment like that - with sun, beaches, sand and everything like that. There are comfy little towns on the seaside and you know that the rich/wealthy/intelligent young people reside there - they all have bright futures ahead of them and still they have the time to fully enjoy life. You see parties, alcohol, fun. Everyone has a great time without a single worry in the world - everyone except you.
As he said: Even a paradise can be a hell if you have to endure it alone - I would even say that paradise (like Isla Vista) is worse when you are alone/miserable than hell (like some shithole), in hell you can at least cope with people having to endure shit all day and also having shitty lifes, in a paradise like Isla Vista this cope is impossible because you see the direct contrast between their life quality and your own (and you make comparisons like all other human beings). Paradise (where life could be so great) is worse than hell when you are miserable yourself. Cannot even imagine how undesirable males cope in Australia/Hawaii/etc. or other chilled places where cool people reside for fun/parties/sex/etc. and where you constantly see that shit and instinctively know: You will never have what it takes to belong to them. Not movies/video games/etc. made him into what he became but reality as it is - no cope from the media or some fancy social scientist will ever change this (not an excuse for his actions of course but an attempt of understanding them).