"We the people of the United States, in order to form a more perfect Union, establish justice, insure domestic tranquility, provide for the common defense, promote the general welfare, and secure the blessings of liberty to ourselves and our posterity, do ordain and establish this Constitution for the United States of America." - Preamble to the United States Constitution.
America is not a utopia. There is no such thing as a perfect society. People will always do bad things for various reasons. The first documented account of violence was between the third and fourth individuals to ever even walk the earth, when Cain killed his brother Abel.