The answer is: elected officials who once elected claimed absolute power.
While leaders of oppressed peoples can sometimes rise to power, they are not typically associated with becoming tyrants. These leaders often emerge to challenge existing power structures rather than to establish tyrannical rule.
Kings who abused their powers can indeed become tyrants, but historically, the term "tyrant" is more commonly associated with individuals who initially gained power through other means, such as election or popular support, and then abused it.
Oligarchs may create oppressive systems, but the term "tyrant" is more often used to describe a single ruler who exercises absolute power, rather than a group.
This option aligns well with the historical context of tyranny. Many tyrants have come to power through legitimate means, such as elections, and then consolidated power to rule without checks and balances, thus becoming tyrannical. Examples include leaders who dismantle democratic institutions after being elected.