In Voltron: Legendary Defender, the five Paladins must defeat Emperor Zarkon to end the threat of the Galra Empire, but they must also face Zarkon's son, Prince Lotor. Lotor isn't the same kind of threat that his father is. Wheras Zarkon is the all-powerful warlord who relies on brute force and fear to control his domain and keep the subjugated races in line, his son is half-Altean. As such, Lotor has very different interests than his father.
The Fairer Villain
Prince Lotor stepped onto the scene in Season 3, and not a moment too soon. His character serves two important roles in the story of Voltron: Legendary Defender. One of them is to provide some much-needed contrast with his father, Emperor Zarkon. Zarkon is a typical "evil overlord" sort of character, whose menace comes from the sheer firepower that he commands and his single-minded mission to conquer the entire universe and crush his foes. He's like a hurricane or a hammer: blunt and powerful. There are no tricks when it comes to Zarkon. The Paladins of Voltron must face this ancient emperor and storm his lair, and then crush him if they can. But it's not so straightforward with Prince Lotor.
If Zarkon were the only villain, the Galra Empire would be rather simple: an empire of brute thugs who want glory and victory at all costs. It was standard villainy until Lotor and his personal bodyguards arrived. Lotor is half-Altean, and his mixed ancestry made him a curious case. His Galra father, Zarkon, wants him to be the supreme ruler and fill his shoes, while Lotor's Altean half makes him sympathetic to the plight of Allura and the fallen Alteans. He is trying to play both sides, and makes friends and enemies from both. That makes him far more dynamic than Zarkon, and makes Voltron as a whole much deeper and intense.
The plot quickly becomes more elaborate as Lotor's own schemes unfold, and he proves that Galra villains can use more than iron will and big guns to accomplish their goals. Lotor wants to recreate the universe in his own image, and he accomplishes it with careful diplomacy, trusting his allies and much more. He wants the Galra to be powerful, but not at the expense of the other races, and his sympathy for the Alteans hints at the possibility of Galra-Altean cooperation and co-existence in the future. Even his own name suggests Galra-Altean unity, being a combination of the Altean Lotarius and the Galran Koltor. He began as yet another threat to Voltron, but his character quickly became a curious case of "what if?"
Good Altean, Bad Altean
Lotor's character serves another vital role in Voltron. He is half Altean, meaning he and Allura have roughly similar interests but very different methods. They both long to see Altea restored and create a proper future to the Altean people. For a time, this common goal united them. They designed and built a powerful trio of ships for Lotor's use, and journeyed together to the mythical land of Oriande, the center of Altean alchemy. Lotor loved Allura, and she in turn got close to him to the point of making Lance jealous.
However, Lotor and Allura are not consistent allies. His role is to show the dark side of Altean ambition, and to show that "I want to rebuild the Alteans" can take one of two paths. Allura's was the path of peace and cooperation, while Lotor's was one of lies, exploitation and power. In that sense, Lotor was truly acting as Zarkon's son and blending his father's methods with Allura's desires. Allura later discovered the truth: the Lotor had set up a colony of Alteans and was draining their quintessence, killing them in the process. A survivor, a young woman named Romelle, explained it all, and Allura and the Paladins turned on Lotor. By now, it was clear that Lotor was on the side of villainy. He represents how Allura might have turned out if she were more ruthless, more desperate or had no Paladins to hold her accountable. Lotor demonstrates what a dark Allura might have turned out to be.