My folk-etymology of Mauvaissoir - since we only meet him in the later 1100s - was some bastardised Old French. He goes about doing evil in the evening:
mal vai [
sic]
en soir. Not unlike as the Devil makes his rounds,
circuit quaerens quem devoret (1 Peter 5:8).
There's a real
etymology for 'mauvais' of which I was unaware. Old Occitan
malvatz points to a bad
fate, not ill will or (in my thought) ill deeds.
Seems there's a story buried in here about how Jehan meets a sorry and ironic end.