I'm pretty sure I'd end up in that gray zone that a villain like Magneto occupies. Yeah he's done some pretty terrible things, but his motives for seeking ever greater power are based on protecting his people so he's not all bad. I don't think it would be worth the effort though to try and actively govern the whole world as a super tyrant though. Too easy to get bogged down in the minutia of ruling a personal empire, I think it would much easier and more effective to let the world govern itself most of the time and just come out of your lair from time to time to eliminate problems.
If I suddenly woke up with super powers tomorrow I'd start tackling big, pet issues right away. I'd apply my own very effective regulation on the financial sector, remove the most toxic personalities from the US political and media scene, curb pollution and green house gas emissions in a variety of industries, stop mountain top removal mining and clear cut logging. My preferred method would be just gather up the people responsible for all these evils and hurl them into space or drop them into an active volcano, assuming I had superman like powers where I just go in fast and heavy and get the job done.
After cleaning up the U.S. I'd work on the rest of the West before worrying about external threats. Finally when I thought the most dangerous internal enemies to Western Civilization were effectively removed I'd expand globally and use my super powers to take out nuclear installations and other sources of WMDs and probably work on some regime change. After that the world could go into maintenance mode and I could just watch out for new bugs to pop up before troubleshooting them.
The only problem with this that in my mind I'd be the great savior and protector of the world (with a very strong bias towards the Western world's ideology and way of life) and a good portion of the population would probably be happy with what I was doing, at least the portion that shares my political and moral outlook. Secular, environmentalist, socially liberal people that long for a competent government that bases policy on actual facts and evidence over superstition and cronyism would be delighted in the changes I'd make (even if grousing about my sometimes undemocratic space ejections.) However religious fundamentalists and other conservative people around the world would view me as a super villain. Also the people I flung into space would probably see me as a villain.
Just like Magneto sees himself as the hero and protector all mutants in the X-Men movies. I think most people would end up like that, a hero of sorts to their own constituency but a villain to others.