This is a fight I've always been interested in ever since I was a kid playing Halo and Starcraft since both factions are nearly identical. The Zerg assimilate other species into the swarm by basically stealing their DNA and all that jazz. Every Zerg creature you see except for a select few like the Defiler and Scourge used to be it's own animal before being turned into the killing machines that they are now. The Hydralisk for example used to be a peaceful herbivore species before wanting flesh and blood like they do now under the Zerg. Pre-Brood War Zerg has The Overmind, the massive entity that controls every Zerg Swarm and also can warp billions across the stars to travel to new worlds to infest. The Overmind has agents called Cerebrates which serve as field commanders. Each Cerebrate is different and provides each brood it watches over with various capabilities with psionic powers. The Overmind can't be killed with conventional weaponry. The Cerebrates can be killed with conventional arms but get reincarnated by The Overmind in a heartbeat. Not even the hardest-hitting Terran and Protoss weaponry could stop these things. If The Flood start to push the Zerg back, Abathur will more than likely develop newer strains of the Zerg we already have seen to be resistant to Flood infection and whatnot. Abathur's sole job is to constantly evolve the Zerg into better and more deadlier forms so it's only a short time before the Zerg will begin to push the Flood back. TL:DR The Zerg win because they have Kerrigan who is a literal God now because Blizzard went full on-anime with Starcraft 2 and ruined the entire storyline and every single character I grew up loving and admiring.