There are several reasons as to why Zaroth decided to use Luna and go all out.
First, he realized that he had been stagnant for too long.
He had become too comfortable, and that comfort never truly challenged him.
After all, if he faced any kind of difficulty, he could just summon Luna, right?
That comfort kept him from progressing, from growing more powerful. As such, when he sensed that there was an opportunity to fight while actually risking his life, he was going to take it.
The second reason was because he desired it.
He knew that David had been hiding his true strength, so Zaroth wanted to make him reveal it and then beat him.
He wanted to beat him at his best.
And he bet that David hadn't told many people if any about his powers—just like Zaroth—so in a way, if somehow both of them survived the fight, it would mean that they wouldn't be inclined to share each other's abilities
