Who's to say that murder and such are America's "real culture"? That seems a direct antithesis to the ideals established in our founding documents. You are the one ignoring those ideals.
It's better to hold "We hold these truths to be self-evident, that all men (in the generic sense, referring to literally everybody) are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness. — That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed," as ideal and fall woefully short (as we often have), than to abandon these ideals altogether. The "real culture" of a nation that holds this up as ideal is one of lofty aspiration. Yes, we fail, but better to fail while aiming for this goal than to abandon that goal for... what? The unbridled genocide and enslavement that preceded the radical notion that all people are equal and have inherent rights?
Do you know when abolitionism in the US began? Not in 1865, but 1776. Many people, including people who owned slaves, admitted the inconsistency of fighting for freedom while owning slaves and found the institution of slavery problematic. It was only a question of how to dismantle it while not simultaneously destroying the US in its very infancy and reverting to everyone being a slave of the Empire.