Board Thread:Questions and Answers/@comment-187.104.5.143-20180214155359/@comment-9782468-20190410122538

A couple possibilities.

Meliodas, after witnessing one too many times his love dying in front of him, finally snaps.

Meliodas has snapped before and we don't know it.

Liz died to Fraudrin, a demon, which reminds him of the Holy War so long ago that cursed them in the first place