I believe the nature of your argument is focused in the wrong area but nonetheless the points you have put across are good ones. Religious teachings do unite people and lead them to co exist and that is a true fact. Religious teachings however don't play a role in how people chose to apply the teachings to their lives: We take a look at Hitler since you brought him.

Hitler was a catholic, one who resented although he remained that way for political considerations, but was only excommunicated prior to his suicide. Catholic is Christian and nowhere in the bible does it teach that we should take it upon ourselves to massacre jews around us, the bible instead teaches us to be Christ-like and Christ never killed any one and in fact tells us to be mindful of our actions to all those around us because they are God's creation. So Hitler had no religious basis to justify what he did but it was simply his hatred for those that he didn't deem pure and those included handicapped people and prostitutes and those that were work shy.

The truth is that people are divided because of differences in religious view but in no way do the teachings discourage unity and co existence.

Islam the Quran states that all humans are created from he same source but with differences. it recognizes the need of a diverse harmonious society for the fulfillment of the societal needs and stresses to overcome: racial and ethnic divides. In no way does this imply that members of the Islam should massacre or perform acts of genocide with other members of this earth.

The problems you brought about are not an issue of religious teachings but simply members of a religion that do terrible deeds because of the nature of their personality and their subjective take on the lessons taught.