When something like this happens abroad, we shrug it off as a by-product of being a less developed country. When it happens here we should think hard why we as a people have sunk to this level.
I am biased but I believe the first cause the divisiveness promoted by the current regime. While we have been tending towards a more us versus them environment, whoever we and they are, the overall example set in many things by the regime has increased this division by many multiples in a short space of time.
Secondly, and I know that many will now brand me as a racist or bigot, I note the location (Southern California) and the ethnicity of the perpetrators (Latinos). Don't get me wrong. I am not accusing all Latinos of being evil or of being illegal. However, California has a high illegal immigrant population which includes a high level of gang related persons and there is a likelihood that this plays a role here.