Spike quickly explained his reasoning by saying, "I have the suspicion that they want to kill him to weaken the world further because everything they've done so far has weakened the world and the Sun King in one war or another."
"At this point, it is normal for me to suspect them of trying to weaken the world because the Sun King has been weakening ever since they invaded. But thrut behavior is extra suspicious this time around because of their determination to kill the Sun King."
"They went to the South Pole in order to kill him as soon as the world became too weak to stop them from venturing out of the North Pole. So I believe that if the Sun King dies, we will lose Aries completely. That's why I think the Sun King should be saved."
He didn't start by stating his true purpose. Instead of being brazenly traitorous, he first linked the situation to his hypothesis about how the invaders possess a world-conquering weapon. Then he made it important for the Sun King to be saved.
