Congratulating him for biting off Zero's head and essentially eating a living being for the first time, the system has given him 3x Devour chances that he can use to completely absorb any creature's powers.
It has been confirmed now that he can even absorb a cursed creature and devour its abilities.
Although Rex is not quite sure how strong the Witch of Chaos is as he hasn't actually met her directly, it's best for him to be cautious and meticulous in every step he takes toward her. With no adequate information at hand, he needs to take a more careful approach lest others suffer the consequences.
Knowing the nature of cursed creatures, the Witch of Chaos is probably very resilient.
The Executor doesn't have the nature of cursed creatures, he adopts an overwhelming power that can triumph over others. If Rex can surpass him in terms of power, then the Executor would lose to him, or at least for now that is what he can deduce from the Executor.