Explanation: The Roman Empire was not a monarchy.

Really! At least for the first 200-300 years. The Emperor was envisioned not as a monarch or some special person of royalty, but as a (admittedly powerful) official representing the people in a republican government. With extremely vaguely defined limits to their authority, and control over the Legions.

... so really it was more like a modern strongman regime or a military junta than a monarchy.

There weren't 'rules' of succession so much as there were a thousand daggers in the dark waiting for their chance to throw their hat in the ring. It's a once-in-a-(imperial)-lifetime chance to become Emperor - wouldn't want to hesitate and miss it!