It's a dictatorship. In a Monarchy, the ruling class comes from nobility. Each person is set into a class-system based around said nobility. The ruling person does not gain power from murdering the King, it still follows a line of succession. If multiple people whom make up a militia overthrow the nobility of a Monarchy and one person receives all of the power, it is then a dictatorship, as there is no line of nobility. As for a dictatorship, this is determined by the circumstances of the gain of power. Normally by a person of a poor background or a militaristic background, the power is taken by force, normally by murdering the current person in power. All of the power is directed towards the dictator, with no other person of nobility to press the "stamp of approval" on each action, nor reprieve the action with a rebuttal, such as the ability with the nobility and the King/Queen in a Monarchy. The power is souly put on one person, if someone gets special treatment from said dictator, sobeit, but it is not appointed as in a Monarchy.