Abathur is an old organism, long serving as an “evolution master” – a guiding hand spinning the strands of zerg DNA into mutations of existing zerg and entirely new strains. Left to its own devices, Abathur’s approach to evolving the Swarm is unorthodox: consuming living tissue, and dissolving it into genetic components able to be quickly reassembled (often painfully).
Abathur’s methods are far from the only means of evolving the Swarm, but the creature has refined them over years of wanderlust following the death of the original zerg Overmind. During its time on Char, the detached, meticulous Abathur modified any zerg it stumbled across, and grew fixated on diversifying the Swarm.
As much as a hive creature can, Abathur sees itself as personally responsible for the continuation of the zerg. It undertakes each experiment as though the survival of the species depended on it alone, and tolerates no evolutionary backstep. Abathur will push the swarm forward…and change it utterly in the attempt.