Haxel had made preparations to sabotage the mission well.
It was a slow but meticulous process that took him years to complete.
Each attempt was profound and would've worked had it not been for Rex, who was an anomaly that he didn't expect. More importantly, each one was prepared well, so there's no chance of preventing it all from happening.
All Rex could do was react and adapt to it.
However, it doesn't mean that Haxel was free from disadvantages.
Having prepared the plan years—before the mission starts meant it's hard for him to alter the plan with Rex in mind. He could only hope—that the sheer depth of his plan would be enough to overwhelm Rex and successfully sabotage the mission.
It was precisely why, even though Haxel knew what Rex was, he made such a grave blunder.
Or perhaps, his confidence came not from knowledge but from ignorance.
