The major downside of any hereditary monarchy is that every so often the throne is inherited by someone completely out of his gourd. Maybe it's the fault of that lead-lined plumbing, or those mercury-laced medicinal elixirs or maybe it's genetic. Whatever the cause, the sovereign is still given the full power and support of the State despite obvious being Crazy to the point he's a danger to everyone around him, with inevitably disastrous consequences.
This guy can compared to the Roman Emperor Caligula, he will be wildly irrational, violently moody, very intolerant of being told anything he doesn't want to hear, and probably afflicted with a god complex. He may indulge in renaming cities or even the entire country after himself. To do anything theEvil Emperor finds displeasing is to inevitably be destroyed. Of course, this could also happen to those who have not done anything at all. Due to their continuing close proximity to the Evil Emperor, members of the Court (decadent or otherwise) will be the primary targets of his fits of rage. The Emperor is very definitely a of a horrible Boss. With any luck, thanks to The Resistance, a conspiracy will eventually develop to remove the crazy sovereign from his post... permanently.
And while all of this is going on, the land over which the Evil Emperor rules is rapidly going down the drain due to his neglect. At best. If he's a Glory Hound, he has an advantage over most: he can start wars.
This is not limited to sovereign heads of state. The Evil Emperor can be anyone wielding great power within an organization while being completely nuts. Modern psychology has shown that the corporate business structure, if emphasizing financial profit to the detriment of anything else, can be especially prone to the appearances of individuals displaying at least a majority if not all of Evil Emperor's traits (so long as the person seeking profits is a psychopath at least, which is what the book seems to be saying.)
The alignment for many Evil Emperors is Chaotic Evil of the Type 4 variety, though some Neutral Evil examples exist. Lawful Evil Evil Emperor are possible (though rare enough to be almost unheard of) as any alignment is subject to madness and Lawful Evil characters do enjoy establishing cruel laws and seeing their minions enforce them.
A real life example was Commodus, His reign was so terrible that historians believe he single-handedly ended the Pax Romana (Rome's Golden Age). Although he was not so much evil as he was power-mad and blissfully ignorant of his responsibilities as Emperor. He believed himself to be the reincarnation of Hercules and personally fought in gladiatorial games - which were always fixed for him to win. He also successfully devaluated Roman coinage, while simultaneously raising taxes, creating a wave of poverty never seen since the days of the old Republic.
Did I forget to mention that this kind of character can make a Dangerously Crazy Sociopath Manchild and a Munduscidal Maniac look completly rational. For Example:
. Hosted is own "Olympic Games" where he won every single event (one event shown being a race where he broke the other athletes in the leg and has the finishing line carried forward to break it)
. Changed a large percentage of the words in the language in his country to "His Name"...including contradictory words like "positive/negative" and "open/closed", causing immense confusion
. Went on a crazed roadtrip in his new Toyota Hybrid and trashing said car not 24 hours after getting it
. Got a massive surgar rush of Reese's Pieces
. Tried to pay an expensive telephone bill by selling his own servant on eBay.
. Makes traitors engage in various, actions such as cleaning in order to receive a pardon, only to laugh at them at the end
. Freaked out at the possibility of getting destroyed while aiding a Duke, sending his Second in Command to do the job instead
. Punched his own second in command in the gut when asked if he (Second in C0mmand) could play his (Emperor) Nintendo Wii
. Responded to one of person's practical jokes using energy weapons, missiles and a satellite armed with a powerful beam weapon
. Goes NUTS when his butler said there was no dinner left on the table, destroying and maiming anyone within a 500 yard radius
. Went travelling the world in order to convince people to eat his own men
. Made their own soliders do ridiculous things to entertain them or else suffer a pummeling
. Invading a planet to build a parking garage or food court
. Order everyone to retreat from battle just because of their snacks being lost
. Blasted more of his own troops in his rages than Foes in battle
. They also tried to kill their best General just for being short