From my limited understanding, the old testament was, among other things foretelling the arrival of the Messiah and showing unbelieving Nations how a detestable tribe (ie, the jewish nation) under Gods rulership could prosper by adhering to his commandments with faith and in turn be a shining light for living by God's word - and importantly what happens if one didn't follow God's prescribed law. So it contains multiple examples of Gods wrath against non-believers, literal slaughter by the thousands etc and prejudicial punishment against an individual. That kinda explains the evil undertones - basically do this and you will be rewarded, if you don't very bad things will happen, either explicitly by God's own judgement or via FAFO. The new testament details the foretold arrival of the Messiah and the fulfilling of the old testament Messianic prophecy. With more emphasis on individual charity, forgiveness and redemption via God's blessings with a view to things that will must also come to pass - ie Satan being set loose upon the Earth and the entire world being under the power of the Prince of darkness.