Why Do Christians Disagree?

Why is it Christians cannot agree to the point where they have condemned each other to hell and/or slaughtered each other because of doctrinal differences that other Christians don't think are essential for salvation? My answer is that Christianity is man made, and as such, doctrinal differences are due to different human ways of understanding the Bible. There is no divine mind behind the Bible, otherwise God should have communicated his will much better than he supposedly did. That's the major reason why there is no doctrinal Christian unity. These different doctrinal understandings continue to be used by denominational leaders to differentiate between Christianities for financial gain and power. Christian, what's your answer?