Fear of people who are different and evolutionary biology
It is an unkown but universally agreed fact that if you paint a monkey green the other monkeys will kill it. It is an actual fact that a chicken which is all strange looking because of folic acid deficiency* has to be kept away from other chickens who would peck it to death.
People don't peck, but they sure seem uncomfortable around eccentrics (believe me I know).
I have a guess. One thing that might make one strange is an infective disease. Killing bearers of infective diseases is an extreme alternative to quarantine which is difficult to manage without hands to build walls and stuff.
I think the adversion many people feel for other people who act different is based on the fear that eccentricity is infectious.
* why am I thinking about folic acid ? see the post below. I got thinking about development and got as far as formation of the notachord which can go wrong causing spina bifida especially if the future mother has folic acid deficiency. This happens two weeks after conception, that is, before you know it, so the rule has to be along the lines of: your choice folic acid pills or the pill.