Okay RECAP TIME! let me help explain everything so far as it's difficult to grasp unless you take notes!
They said "Devil of melancholy" but that was a bad translation, Hector is actually a warlock, so he's a male witch of sin.
By now you have met all the witches of sin, and lets review their current status. We knew in season 1 that satella killed all the other witches:
Echidna (Greed) - Dead
Minerva (Wrath) - Dead
Typhon (Pride) - Dead
Sekhmet (Sloth) - Dead
Carmilla (lust) dead
Daphne (gluttony) - Dead
Satella (Envy) - Alive but sealed away by the powers of the sage, dragon, and sword saint 400 years ago, yet she still has influence over the world clearly cuz of the whole subaru situation.
Pandora (vanity) - alive befcause emilia's flashback took place 100 years ago and pandora was there as you saw last episode. Supposedly satella killed the other witches 400 years ago yet pandora is still alive 300 years later and showed up in emilia's flash back.
Hector (Melancholy) - unknown, he is alive 400 years ago as you see him and echidna talking, and the whole point of putting the barrier up was to protect echidna and the sanctuary from him. WE have no idea why, it might come up in a later arc. I'm assuming he's dead because echidna died by satella, which means she should have survived this battle and died later, sometime after she was experimenting on making ryuzu clones and stuff which all came after.
Also, alot of us think that hector died or maybe fused with roswaal, because roswaal had 2 blue eyes in the past, and now current day he has 1 yellow eye. He also dresses like how hector used to dress which is the clown outfit, and also talks like how hector did back then with exttending his words. Alot of us think they fused.
Keep in mind right now you learned ryuzu's side of the story only which ends when she goes in the crystal, we still need roswaal's side, beatrice side, and echidna side to know the rest.
So basically... re zero will be a 300-400 episode series lol. You guys will learn one day like the rest of us hahahah