I'll start with Cesare Borgia (above), a violent psychopath whose senseless wars of conquest helped to undermine the Italian Renaissance. Orson Welles played him in a film called "Prince of Foxes" which I highly recommend.
How about Genghis Khan (above)? He waged brutal, senseless war just for the fun of it.
Then there's the mass murderer, Stalin, shown here in the days when he was a young thug.
Here's (above) Ivan the Terrible, a man who was aptly named.
Then there was Mao, apparently the greatest mass murderer of the 20th Century, stacking up a body count that exceeded even that of Hitler and Stalin. Read about what went on when he tried to enforce his "Great Leap Forward."
By the way, Hitler certainly deserves a very high place in this rogues gallery but I left him out because I couldn't find a suitable picture. Every photo of him has been seen a gazillion times.
Not so common is this recent reconstruction of the head of Robespierre, architect of the French Revolution's "Reign of Terror." I always pictured him as having a thin, angular face, but I guess I was wrong.