Having watched 16 in full now I have to ask, what does everybody else think caused Leo to put on the mask and start on this path? Possession, split personality, just plain loco.
Whatever the reason it has to do with the Makai brush in he's carrying around and the woman we saw briefly when he was talking to Karou early in the series. Possibly a sister or a girlfriend who was killed while assisting a Makai Knight in battle.
Of course another question has to be how many Makai Priests are going to agree with him and stand by his side?