Leaders must maintain order, even if by force?

This, Miami Herald op-ed has some interesting thoughts. The key premise is:

Whoever aspires to exercise power should know that the first responsibility of a ruler is to maintain the public order and enforce the laws. If he's not prepared to take on that uncomfortable task, even at the risk of bloodshed, he should devote himself to something else. Mesa, for example, was an imaginative historian and a keen journalist. He should not have left that medium. He was not fit to govern.

It is specifically about the turmoil in Boliva, about which I am not up to speed enough to comment on. While I agree that it is the sad duty of the state to use violence when necessary, and political leaders must be willing to employ violence as necessary and appropriate, it is also apparent that this is a slippery slope. How does one determine whether the Government a person is leading is legitimate enough to use violence in it's own defense or not? On the surface, based on this op-ed, Carlos Mesa should have been willing to employ violence against lawless rioters, even those who disagree with this specific case can doubtless come up with scenarios where this would be true. On the other hand, I deplored the actions of Islam Karimov in Uzbekistan. Some of the facts of the two cases are different, some are the same. Obviously, the initial legitimacy of a government, which I feel can only be gained by consent of the governed, plays a big part in this. The nature of the opposition, whether peaceful protesters or violent thugs makes a difference as well. A less fuzzy means of evaluating this seems needed however, and I will have to think on it some.