I have a Crack theory on Maegor's genes.
Visenya gets her sperm for baby Maegor the Cruel from Conquest. How does this happen? Well you see that Aegon has a lot of trouble making his sister wives pomegranate. So he used a glass candle to summon the gods for aid, amongst his many other attempts. Somehow Thula gets the signals and came down to Planetos for a chat and an easy planet to take over.
Thula brings over Conquest to... well, milk the man and magically inject the material into Aegon and then into Visenya, so she can make a strong super mighty soldier baby. For what in return? To get whatever treasures hidden in Old Valyria. People are too freaked out by the place and won't be close enough to see two Viltrumites flying around. And if people see them, they will just chalk it off to Asshai magicians doing something.
This is all a plan to find compatible species to breed with Viltrumites. But Maegor is an infertile hybrid. Think a super mule that can ram your house down. This is why he goes crazy and kills people as a solution. Because that is the only answer he has. Everything is a vase if you are Zelda, as the saying goes.
So all Maegor's cruelty came down from Visenya, who received instructions from Thula on how to be a mighty Viltrumite Conqueror. Also, Thula bitchslapped Balerion the Black Dread to obey Maegor.
In the end, Targaryen is a puppet regime for Viltrum, the Essos is to be conquered after Dorne, and the rest of Planetos will be subdued, with help from Viltrum, and Targaryens will be their vassals. Or that was the plan until Viltrumites see how failed an experiment Maegor is.