It is hard for us to admit that some people commit murder simply because they enjoy it.
There are many reasons people murder others. Simple stupidity, not caring about the safety and well being of anyone other than themselves, jealousy, and the perception that they are not getting their fair share and have some type of victim status were the likely explanations for most of the murders that I responded to. But yes, I think that you have put your finger on the perception of the Manson clan that probably disturbs many people the most.