Why did Meliodas betray the demon clan?
Disclaimer; As of writing this, we don’t have a fact supported answer but I’m hoping to arrive at the possible answer based on the history.
If there’s a weakness that Meliodas then it’s boobs and booze! Jk! Meliodas lost his cool when he witness Liz’s death in front of his eyes. He lost his shit when people attacked Elizebeth.
There’s a high chance that his loved one was among the Godess clan and chose to betray his own clan in order to protect her. This is the most likely explanation to why he betrayed the demon clan.
#2 Pointless War
Meliodas is kind, there’s no denying that. Maybe he was concerned with the loss of lives during this war and hence decided to give it up, so that the world remains at peace. The chances of this are unlikely but let’s not completely ignore this.