Efad was already halfway through the mission he was on. He had cleared the 9th Void Vortex, leaving him three more to go.
Naturally, he had become so strong and so experienced in using the void that, if he became a god, Klaus and his other incarnations could confidently challenge even Primordials.
Klaus wanted that because even if he somehow could take on a Primordial with help from his friends, it would be a difficult battle, one that Klaus was not ready for.
The gap between a Pseudo-Primordial and a Primordial was vast, and so he could not stand and say he could easily challenge and take on a Primordial.
He may have killed two Pseudo-Primordials with ease, but Primordials were a different story.
Aside from that, if Efad became a god, the Star Monarch bloodline would see a huge leap. He was guaranteed to awaken his own Dao, which, when he did, would allow him to easily awaken the Primordial Grade bloodline for his Beasts and everyone he gave his blood to.
