At last, the thread of my hopes and dreams.
Lets see…As for the most evil thing I could do TODAY, I could probably torture to death someone who had hurt someone I cared about. Probably even assassinate them in a courtroom if I felt they weren’t being punished enough. (By the way, SteveSteve, I have just one thing to say to you…If you’re torturing someone, try screwing around with their Cochlea’s and Inner Ear as well. Maybe you could leave them with perpetual tinnitus and nausea as well.)
If I were something like the unquestioned lord and evil master of the Earth, I think I would…
Arrange to have all the disputed holy sites, of ALL religions (I’m an equal-opportunity evil overlord), carefully dismantled, brick by brick, and reassembled, brick by brick…at the bottom of the Pacific Ocean. Just out of pure spite. Fight over 'em NOW.
Start using Vlad the Impaler’s methods of criminal justice and punishment. Over the course of 48 hours, starting with murderers and sex offenders, and moving to the other felons from there.
Use nuclear weapons to carve state and provincial borders into the Earth, so they can be clearly visible from orbit. I’ll dump the contaminated soil on my least favorite continent, spelling out my name.
As for what I could do with TIME TRAVEL, well…
Go back in time, Agent-Orange Teutoberg Forest, give gunpowder technology and the beginnings of electrical technology to the Romans.
Go back in time (Bringing necessary tech. to back this up) to the Aztec Empire, a couple of years before Cortez lands. Use a jet-pack to fly about the capital city for a little while, then land on the top of the main temple. Tell the Aztecs that I’m an emissary of the sun god, sent to help his people, and that a false emissary bringing death and blasphemy was soon to arrive from across the seas. Proceed to give the Aztecs vaccines for assorted European diseases, and enough weapons and industrial technology to bring them up to a late 18th century technological level. Then sit back and watch the fun begin.
Go FORWARD in time, steal a huge, two-legged battle 'Mech, and use it to periodically show up through periods of Earth’s history and let loose my inner juggernaut. Then spend a relaxing weekend at the library, looking through the altered history and theology books, and laugh my head off.
Anyone want to be my friend? 
Ranchoth