Many times Jesus quoted the old testament, saying "It is written..." and correcting or clarifying it, saying "... but I say unto you." For Jews the old testament is the final authority. For "Christians," Jesus is. If you are a Christian, Harry, remember that whenever you find conflict between the old & new covenant. Jesus advocated living as one, but you're right that many advocates of one-world-government are not interested in oneness, but in control over all by themselves. I would not want to be ruled by any of the world figures present today, be they Trump or Putin. 
Power corrupts, whether in city hall, or the White House, or the Kremlin. George Washington was right that "government is not about service -- it is about power." Only when men have the heart of service to all that Jesus spoke about will we have a chance at world peace. Unfortunately not even Christians, who profess to call "the Prince of Peace" are willing to refuse to fight in war, even against other Christians.