The problem is that you do not occupy a privileged ontological position. You may seem mentally ill to someone in the future, who has a different set of morals. There are plenty of other people in the history of the world who have thought that what they were doing was the best for humanity, or at least the part of humanity they cared about. If you really believed that disbelieving in God got you sent to a place of eternal suffering - and even worse, if you believed that disbelief could be caught from heretics - then you would be acting in a perfectly virtuous way in trying to stamp out that sort of heresy with violence. If you really believe that drugs ruin lives, if you think the world functions in that way, then you are acting in a perfectly virtuous way in operating the war on drugs.
Of course we can note a lack of evidence, or even contrary evidence, in both of those cases. That's not the point, the point is that that's just how reality looks to people in those positions and you have no reason to presuppose that you are not sharing similar errors in whatever you would decide on as "right for all of humanity." You will even find disagreement over what people believe constitute "basic human rights" you will find places, and times in history, where your right to life would have been – and still is in some places – a laughable concept. And even today you find certain rights that seem to contradict other rights, I'm reminded of the fact that you have a right to an education in Germany but that this right has been interpreted as a mandate. Which rather seems in conflict with my right to pursue happiness, my right to be free from imprisonment, and so on. (Granted, people generally don't treat children as having many rights - but since we're talking right for all humanity.) Similarly there are great number of rights that you would think people have but that for various reasons they do not – or at least have no functional way of asserting. My right to control my reproductive system for instance - I've found it almost impossible to get my tubes tied on the NHS because doctors continually tell me that I'm too young to make those sorts of decisions about family. (Or, in one rather vexing case, outright asked what was wrong with me.) Despite being a woman in my twenties who reacts extremely poorly to birth control (i.e. it makes me a crazy bitch). They interpret a right to a family in a strongly positive sense - they've decided that's right for all humanity.
The sort of power you're talking about would ultimately be wielded by politicians, by psychologists, by the same sort of doctors who make decisions about what people can do with their lives and their health who have made terrible decisions in the past and continue to make terrible decisions today. Do you trust those people? The same sorts of people who decided that being gay was a mental health problem? Even if we accepted that you could get together a small group of people who honestly did not desire to be tyrants, even if we accepted that you would only define those things that seem to us on the basis of our current morality as the most egregious examples of violations as mental health problems, there is no one worthy of giving that sort of trust to. That's probably how the situation looks to everyone who ends up as a tyrant.
They have misused that power in the past, they continue to miss use it today, and you have no reason to suppose that they will not miss use it in the future.