Saleen destroyed all three weapon spirits without a second thought so he could use the Lightning Moon to recreate a weapon spirit without memories or enmity against him.
The Lightning Moon was so powerful that not even he, as its master, would have been able to endure more than a few attacks.
The Goddess of Myers never expected Saleen to be greedy enough to destroy those weapon spirits once they became demigods. If he had kept them around, they would have been traps the goddess put at his side.
The more Saleen interacted with ancient humans, the more he understood their reasoning and principles. Even though he shared similar ideals with them, they would have had no qualms about using him as cannon fodder.
It wouldn't have mattered how careful he was with everything around him. Myers made the three weapon spirits side with her, and the general put 24 contracts out on him. If he mishandled any of them, he could have gotten himself killed.
