Nobody wants to be a manager ; everyone wants to be a leader. So let's compare the two. A leader is a visionary with grand goals in mind ; the manager meets deadlines. The leader has scores of people worshiping him ; everyone despises their overbearing manager. The leader inspires ; the manager dictates. The leader takes bold risks ; the manager maintains status quo. And so on..
TED talks and innumerous self help books will convince you to be a leader rather than a manager. Even MBA schools don't really manufacture managers ; they attempt to mold the "future leaders of the world".
So why is the manager an ass ? Is it because that managers are inherently evil people and are attracted to that job profile ? I would argue that it's simply a case of "long-term" vs "short-term" thinking. What's good for your company in the short term isn't necessarily beneficial in the long term. On the other hand, if the world was full of long term idealists, nothing would ever get done. In nerd speak - Attempt the local optimization problem in order to solve the global one.
That's why I loved the movie Lincoln. It shows tension between leaderliness and managerliness. I shall leave you with a quote from the movie -
"A compass, I learned when I was surveying, it'll... it'll point you True North from where you're standing, but it's got no advice about the swamps and deserts and chasms that you'll encounter along the way. If in pursuit of your destination, you plunge ahead, heedless of obstacles, and achieve nothing more than to sink in a swamp... What's the use of knowing True North? " - Abraham Lincoln