"Demagogue" has a bitter taste to it by definition, but it doesn't have to be this way. Wikipedia lists four in their article about the word, and none are positive examples. Merriam-Webster has two definitions. The first echoes the Wikipedia page, but the second seems more fitting. "A leader championing the cause of the common people in ancient times," seems closer to my belief. The roots roughly translate to "lead people."
A demagogue is someone with the ability to empathize with people and convince them that something needs to be done. Demagogy is what public speakers mostly wish they could accomplish, and what the successful ones do achieve. It's a skill that's half learned, half inherent. It's a tool for leadership, like philosophy and psychology. In many cases, it's incorporated into this ability when cemented with diction and passion. Maybe that's why all the people listed on the page are such terrible human beings.
I learned the word from a game a few years back (your vocabulary increases exponentially when you play text-heavy games, it's amazing), it was a character class for some terrible table top platform. Upon hearing the ability, I jumped on it. The ability to sway minds and hearts with words is not only powerful, but I'd been learning it since early on in scouting. It was something I'd used in the past to save lives, organize people, and spare people from bodily harm. It's something wonderful that should be more common amongst people in leadership positions.
Demagogy isn't a key to limitless power, nor is it a promise of being able to change the world in a huge way. In some famous cases - Barack Obama most recently - it does. In most cases it's an underdeveloped ability that doesn't catch on most that can make use of it.
It isn't always used to positive effect. The most famous demagogue in history is Adolf Hitler, a violent madman responsible for unfathomable death. He never used force to get to power, he only used words. He believed what he was saying, and he knew how to and which strings to tug to convince his audience that he was telling the truth. When you tell a lie often enough, you start to forget the truth. After a long enough time, there is no more truth for you. People do this all the time, but most people don't have the ability to change minds. This is why it is so potentially dangerous. A demagogue could be anyone.
To the previous example, Barack Obama has a similar ability that gets used to a more positive effect so far. His skill is more learned than genuine, but the skill is still there. The speeches aren't particularly powerful, but the choice of diction is clear and concise. He chooses concepts that are important enough for his audience to hold on to, and tells them the things that he wants them to hear and know. While there are other clauses to most of what he says, he doesn't present them in the same way. No demagogue ever does. They must gather the consent to do what they plan to do, then execute it after. All politicians do this, but most aren't honest or genuine. These are two very important traits in any leader, and for an effective positive demagogue they are the most important traits one can have. There are other outside factors that relate to his presidency, but most have nothing to do with the ability to lead.
This can only exist if there is honesty in the world, because honesty is key. You must be trusted, and be able to trust yourself. You must not lie. You must be able to tell what you believe without fear of judgment for it, because you will be judged. You will be torn apart and criticized. You will not be well received by everyone, but the people that do will welcome you warmly. If you lie, people will know.
This can only exist if there is genuine care for the well being of the people you care about. You will not get anywhere if you're lying and working solely towards personal gain. You don't do it for money or fame, you do it to ensure that the lives of others will be enriched.
In my relatively short life so far, I've been in leadership roles many times. I've worked to help students, scouts, and a number of other young organizations that need voices. I care enough about my peers - even the ones I don't like - to fight for them. That's what a demagogue does, they do what they believe is right. The only thing more important than doing what is right is making sure that what you believe actually is.