It wasn't a test initially.
But it is now.
Haxel had prepared to sabotage the mission from the very start, even before he knew Rex was appointed to make sure he didn't succeed. He prepared each attempt meticulously and made it so that the attempt got harder the closer they were to the end of the mission.
He second-guessed himself upon making these attempts.
Perhaps, he should make only one attempt that'd decimate the mission so fantastically that there would be no coming back for Empress Morgana. Or perhaps he should lower the destruction, as he didn't want to go overboard and incite the empire's full attention.
Due to those thoughts, he decided to make multiple attempts that gradually increase in damage.
