I think it is entirely reasonable to expect a dictator (or powerful figurehead) to rise on the promise of reform and then turn around and consolidate military power instead of tending to more important domestic matters they promised to. That seems to be a pretty regular theme in real-world political history.
And from the very beginning, including TEA, it has been emphasized that Heth is a politically naive man with a very narrow set of agendas.
This is somewhat of an accelerated timeline so far, but not altogether an unlikely chain of events.
