Maou-sama and Ken Izumi are not so different - in a way. Whereas Ken is the defender of the human race, Maou is the defender of the Juralians. His speech's wording (or, rather, the translation) in Episode 47note specifically, "As soon as possible, we must call together our dying bretheren to conquer the Earth, and reclaim peace and prosperity for our race once again!" implies that some great tragedy happened to the Juralians and Maou led them to the Earth so they could have a homeworld again. However, Maou's ways of achieving his goals are ratherviolent... just like Ken himself, who kills nearly every Juralian just to halt their invasion. And, as shown in Episode 11, when a threat (the Iron Star) that would destroy Earth comes, Maou and Ken team up.
It is heavily implied that the Juralians have completely infiltrated society on a few occasions. There's the plane hijacking in Episode 37, where everyone, except Ken, his family and the hijackers, turns out to be Juralians. Another episode features a man marrying a Juralian disguising as a woman; the first thing she does is to plant tiny bombs (or something) in his pockets. And finally, there's the piano teacher from episode 45, also a Juralian who possesses Caron, Ken's usually sweet sister, making her try to stab him with a knife in the middle of the night — and almost succeeds. Needless to say, she has a My God, What Have I Done? moment once she gets the possession slapped out of her...
A Fridge Horror on that: the Juralian disguises were so effective that it is outright impossible to tell whether the Juralians were all gone after Maou was defeated. And even if they were... Who's to say that Ken, the ridiculously overpowered protagonist who can kill in pretty much one shot and also punched out a trained boxer with his bare hands, would ever believe this and stop looking for Juralians to kill?
Based on the above, imagine yourself as a Juralian.
You are a Juralian. Your people are dying. Your home planet is most likely destroyed. Your king is a warmonger and your people are fanatically devoted to him because he's your only savior. You have been here for 50 years, which gave you time to infiltrate the society completely.
Now, your king wants you to kidnap all the human children in the world to use as hostages. But look! A human preteen is slaughtering your fellow soldiers easily. All your schemes are foiled by this preteen boy as he willfully kills your people. But your king continues to send you to your death. If you object to your given task or defect, your king will have his cronies hunt you down and kill you. When you ask him to do something about this "Chargeman" at a meeting, your king responds with a mere "Never mind!" and says that the Chargeman is no threat based on the assumption that humans are weak,and so is Ken. Then he sends yet another batch of your fellow soldiers to their deaths.
Then comes time to launch a full assault. Finally the Earth will belong to the Juralians! Whoops! The Chargeman has destroyed lots of your spaceships, found the location of your hideout, and killed your king. You know, the one who sent you to your deaths in order to claim Earth for your race. Now that he is dead, you no longer have a reason to effectively kill yourselves, but you can never live as a Juralian, because the Chargeman, a paranoid, murderous psychopath, will hunt you down and he will be the good guy. As you live the rest of your life as a human, you are constantly on the run, because most likely you don't have a passport or a diploma showing your level of education, meaning that you are jobless. You may never find out who's a human and who's your fellow Juralian because you both can't reveal yourselves. You may experience romantic attraction to a human, but she/he will find out what you really look like. You will never be able to have a child, and even if you have one, he/she would be a hideous abomination. You will die a paranoid, neurotic man haunted by nightmares of the Chargeman and the fallen Juralians, and you most likely wouldn't be able to do anything about it.
When it comes to episode 53, "The Suspicious Bride"... It begins innocently enough with a wedding, and later returns to the happy couple some time into the relationship. All kinda sweet—until you realize that the "blushing bride" is a one-eyed, tentacled, red-skinned alien in disguise, and the groom had no idea. Did I mention these lovebirds just came back from their honeymoon? Fortunately this is a kids show, so nothing is mentioned of the possible side-effects...
Episode 35, "Dynamite in the Brain", particularly Dr. Volga's death. While Dr. Volga was shot by the Juralians, it is unlikely that he died immediately. Combined with Ken's notoriously rash thinking, there is almost no indication that the Dr. Volga that appeared at the reception was, in fact, a "human robot".