The destroyer was held over Rowan's head like an executioner blade, and the moment Thenos was within reach, Rowan was taking his head once more. The infant titan was not used to commandeering its posers and was mostly using his aberrant ability from the eyes on his head, if Rowan could decapitate him, then his offensive potential would become nearly null.
All of these actions needed to be performed quickly for the mind of a higher dimensional being was incredibly astute, and the poisonous nature of the darkness that was corrupting the intellect of Thenos would not remain for long.