What Pushes a Man from Activism to Revolution?

What is it that moves a man from activist to violent revolutionary? Where is the very fine line between the two located, and what pushes people to - and over - that line?

I kept thinking about these this weekend as I watched a series of films on Cuba, the Cuban Revolution, and Che Guevara. The questions seemed particularly relevant as I was watching "Motorcycle Diaries", the story of Guevara's 1952 journey across South America with his friend Alberto Granado, based on both Guevara's journals and a book written by Granado. During the course of their journey - especially in Peru where, following the breakdown of their motorcyle, they were forced to accept rides in trucks - they came in direct contact with a significant number of people who, regardless of their country, were subjected to extreme poverty.

A husband and wife who left their children with trusted friends so that they could travel to find work in the mines. A Peruvian farmer who banded together with other farmers to help each other with their crops and plowing, and who was also forced to move in order to earn money to send his children to school. A hospital devoted to the care of those suffering from leprosy, but which was split in half by the Amazon River - patients on one side, staff on the other. Indigenous people who, as Guevara noted, were refugees in their own country.

All of this changed Che, and he became intensely interested in helping to change a system were people were faced with injustice every day and where (and I cannot recall where I read this) tomorrow is the only horizon these people can see. But what happened? How did he go from a physician and someone who was devoted to helping people overcome the obstacles in their lives to one of the leaders of a nation which executed thousands of political opponents - many at his own direction?

This, of course, brings me back to my original question: what pushes a man over that line, from activist to revolutionary, doctor to murderer? Guevara's life is already an enigma - a saint to some, a terror to others. Two sides to the same coin, perhaps - hero and villain? His entire life seems divided by that fine, but very visible line - but what forces moved him, and many others throughout history, across that divide?