Not to say they don't kill each other. They just do it for other reason. Survival. Humans on the other hand, is so obsessed with their self image they forget who they are. They have enough food, but the first thing they do when they wake up in the morning is looking in the mirror.
And humans doesn't only define themselves, they define each other too. For whatever reason, they devaluate an image every now and then. If you happen to have that label, your in deep shit. Everybody is out to kill you.
There's no logic to it. Suddenly a colour comes out of fashion. An origin or heritage becomes unpopular. An age, a gender, a condition... you name it. Mostly anything under the sun is up for grabs.
To distract attention from this horrible caste system humans tell themselves you don't choose your parents when you born into this world. Truth is, you don't choose your labels.