Whatever the touch of Thenos did that deprived Nemesis of its energy, Rowan would not give it the chance to regain it back. Nemesis was an eighth-dimensional entity, that was even more powerful than Gothran'Inul when placed in its center of power which was the Great Desert, outside its domain and restricted from accessing its greatest weapon by the touch of Thenos, Nemesis became the juiciest prey here, and if Rowan's siphoning of soul energy from the corpse of Thenos were to be interrupted, then he would need fuel to push him through the upcoming battle, and Nemesis soul was to be that fuel.