WARNING: The following contains spoilers for Young Justice: Phantoms, streaming now on HBO Max.
In the first half of Young Justice: Phantoms, fans were stunned by the backstory of Vandal Savage. He initially cut a somewhat sympathetic figure, revealing he did have humanity's best interests at heart as he wanted to create an evolved race of superhumans to make Earth one of the alpha planets in the cosmos. Unfortunately, thanks to Klarion the Witch Boy intervening, Vandal got caught up in a series of wars to please the Chaos Lord, which led to the plan becoming muddied a bit. However, thanks to the battle against the Child and a key convention with the Lords of Chaos and Order, it became clear Vandal was way scarier than the Thanos seen in the Marvel Cinematic Universe.
Now, this isn't to disparage the MCU's Mad Titan, as the villain was truly terrifying. He enacted a years-long mission to acquire the Infinity Stones, using the Infinity Gauntlet to wipe half the galaxy away. Thanos even killed his daughter, Gamora, to achieve success, but he did have somewhat sound reasoning.
Overpopulation was a crisis that destroyed his homeworld, so he wanted population control, leaving more resources for the survivors. In his warped vision, it was all about course-correction and balancing, crafting a healthier universe that'd heal in time and appreciate the sacrifice.
Vandal's mission isn't as altruistic when deeply dissected. The wrinkle in his so-called humanitarianism came when he told the Chaos and Order Lords he wanted Earth to become a powerhouse as he predicted there'd be one final war between the cosmos' last two planets: Earth and Apokolips. As such, Vandal was willing to cull Earth and use tools such as the Light, Warworld, New Genesis and even Darkseid to purge the galaxy and make his army stronger.
All these wars, against the Reach, and even the terrorism on Mars, were trials as Vandal kept shifting things up on the chessboard. He didn't care how many other planets died, as long as everything served his purpose. Even if he created a stronger Justice League through massacres, he'd get what he wanted. It's why he didn't want the Child decimating Earth, begging the Lords to reduce her power, so his hub would stand a chance of winning and become as strong as possible for the clash of the heavyweights to come.
Since the comet hit and turned him from a caveman into this immortal puppet master, it's been his destiny, leading to him viewing himself not just as a warrior, but as a god leading this final charge. Rather than leave half a cosmos behind, Vandal, who has always believed in "only the strong will survive," is adamant only one winner can claim the prize. Thus, he wants it whittled down to its most minimal state so that these predators will then procreate and make a stronger reality.
It's a vicious way of looking at the cycle of life, made even more sick by Vandal accepting if he loses, the galaxy will be better off under a stronger, deserving Darkseid. Ultimately, he isn't about recovery -- he's about rebuilding, which is a far cry from the benevolence and mercy Thanos sometimes showed.