If all Heydrich needs to break the timeloop is a massive amount of souls, wouldn't it make more sense to add some politicians, engineers, and scientists to the Obsidian Roundtable? He and Mercurius could imbue them with occult knowledge and they could get to work influencing nations' policies, eventually leading to a one-world government. Such a government could have unlimited power to build human breeding farms and cloning facilities throughout the world and harvest people's souls en masse. That seems more sustainable and controllable to me than random killings. Obviously he'd still need some warriors and battles to trigger the Swastikas, but focusing entirely on war seems very narrow-minded to me. Heydrich's politicians could even incite infighting and border conflicts between provinces so the can warriors still collect strong souls and stagnate in power. Or maybe people without any fighting prowess get absorbed into the Castle too quickly for such plans to work.