Board Thread:Questions and Answers/@comment-13439417-20151115150226/@comment-26091443-20151118200419

It's very possible he was but I think it's going to turn out he was a more idependent warrior who happened to work closely with them.

Meliodas isn't human so I don't know why it strikes you as weird that he doesn't age like one but he also apparently doesn't age or have the lifespan of a normal demon so who knows what's going on there.

And yeah, it's very clear when they're calling him a traitor they mean to demons in general. Galan has outright said so. It wouldn't make much sense for them to be calling him a traitor in reference to them specifically.