I don’t think people who find things in the Old Testament and say, “Looky here, that’s a sin” fully understand what it means to be Christian. What’s in the Old Testament is just a bunch old Jewish laws. Christians are now ‘above’ the law. Consider that Jesus Christ was completely without sin and yet he frequently and flagrantly broke many of the Old Testament laws. Sinning is not so much what you do but why you do it. Love God more than anything and love your neighbor at least as much as yourself. Do those two things and everything else is pretty much optional.
If you are religious, you believe that your religion is the 'right' one—and, in many cases, all others will be sent to hell. Similarly, a nationalist believes his or her nation is better or more advanced—and a racist believes that an inherent difference between each race make his or her ethnicity superior. All of these ideologies spawn the hate, philosophical disagreements, and prejudices that have been the catalysts for various atrocious acts throughout history.