There is this part from Vol 32 extra "When I heard that you betrayed the demon clan and joined Stigma, even while in doubt, I had hope that no matter how things got this way, if the successor of the demon clan was on our side we could actually win the Holy War But what happened in reality?! You were punished by the supreme deity and the demon king and disappeared somewhere And then all of a sudden Estarossa surfaced when you were gone And then Mael-sama was killed!! That person was the goddess clan’s… he was my light of hope!! I wanted to serve by his side longer! Even if my feelings didn’t reach him!" It seems Mael got brainwashed after Meliodas and Elizabeth got killed and cursed.