I don't think there is a single common English word for OP's context. A motorbike or car, for example, might be overpowered or have an overpowered engine, but overpowered people would be people who've been overcome/defeated, not people with too much power.
Not so common, but easily understood in context, is over-empowered.
Note that in practice, the problem OP seeks to highlight normally either reflects a systemic problem (OP's "excessively powerful person" is part of a system lacking appropriate checks and balances), or that person has been overpromoted (he lacks the character attributes that would enable him to exercise his power without raising concerns among the people subject to his decisions and actions).
There are of course a whole host of words to describe people who have and exercise excessive power, but they're normally somewhat "politically loaded". For example, dictator, Fascist, [little] Hitler, authoritarian, despot, overlord, totalitarian, tyrant, etc.