Perhaps the word you are looking for is Cu De Ta? Or, Coup d'état.
(pulled from wiki)
A coup d'état (pronounced /ˌkuːdeɪˈtɑː/, us dict: kōō′·dā·tâ′), (plural: coups d'état) or coupfor short, is the sudden unconstitutional deposition of a government, usually by a small group of the existing state establishment-typically the military-to replace the deposed government with another, either civil or military. A coup d'état succeeds when the usurpers establish their legitimacy if the attacked government fail to thwart them, by allowing their (strategic, tactical, political) consolidation and then receiving the deposed government's surrender; or the acquiescence of the populace and the non-participant military forces.
Coup, a putsch, or an overthrow and also know as a coup d'etat is seizure of power by force. It is considered successful when the usurpers establish their dominance.
Usually (but not necessarily) by force. But in any event, not by statutorily legal lines of succession.
The unjumbled word is "usurp." It means to take a position of power or importance illegally or by force.
Both. He was elected but he killed people who opposed him and elected his own men in.
The word you are looking for is "seize."
Seize means to take possession by force.
greece
There is no term for a bad leader, but you could say a tyrant is a bad leader because tyrants seize power by force.
seize
To drive or urge with force, or irresistibly; to force; to constrain; to oblige; to necessitate, either by physical or moral force., To take by force or violence; to seize; to exact; to extort., To force to yield; to overpower; to subjugate., To gather or unite in a crowd or company., To call forth; to summon., To make one yield or submit.
Iran
In September 1917
The Bolsheviks did not seize power during the Russian Civil War. They had already seized governmental power from the Provisional Government in 1917. The civil war is generally figured to have started in 1918. The Bolsheviks managed to retain their power, not seize it, by winning the civil war.