I was gonna say Bush, but since our Nicelandic friend already mentioned that

..
http://www.historyguide.org/intellect/robespierre.html
Robespierre, man of, as I heard, Irish origin. He started out as an extreme royalist, but his popularity later grew.
He was the dictator of France for a very short period, but it was he who made much use of the guillotine, and by the sources of my history book, sent approximately 40 000 people to execution.
I sure hope I got my info right, since this link doesn't say much about how he started the terror politics. My history book regarded to him as a very cruel man.
Here is a cute picture of him:
http://www.historyguide.org/images/thermidor.jpg
My history book had this picture, and a note close to it:"This is Robespierre executing the executioner after killing of the rest of France.
This guy may be too old for this topic(1758-1794) since you mentioned tyrants of a much nearer past
I also wanted to mention Gengis Khan, but he's got a mustache