Role of the military is to uphold democracy, not overthrow it 

There is no such thing as “democratic tyrants”. If they are tyrants, then they are not democratic. Tyrants can be voted into power but not in a democracy. There is far more to democracy than suffrage, the right to vote. Suffrage is a principle of democracy. Democracy is a system of government controlled by laws, not people.
Any person voted into power within a democracy is constrained by laws that impose accountability and control behaviour.
A democracy must have strong but fair law enforcement with an institutionally apolitical judiciary and an institutionally apolitical police force.
This is the failing of Thailand, preventing it from being a democracy. It has had 20 coups since 1932 when suffrage was introduced. The rich run free while the poor are incarcerated for trivia.
The current administration in the United States is faced with a failed democracy because the former administration weaponised the Department of Justice (DoJ), the CIA under John Brennan and the FBI under James Comey. 
They attempted to prevent the Republican Party gaining power by corrupting the intelligence agencies and the DoJ to spy on the Republican candidate, Donald Trump. This is currently being exposed with documentation.
Democracy only exists where all people, rich and poor and those in authority, are subject to the laws of the land. It is not about being voted into power but about accountability under the law.
J C Wilcox