Jowan was jealous of the wardens power, its why he sort out the book of blood magic, to increase his own power, nothing noble about that. I wonder if he would have butcher Lilly for power if they had escaped as planed and templar’s were closing in.
I'm currently replaying Origins as an Amell mage. From what I gather, Jowan, and not even the real one, makes only a single implication of jealousy and that's in the temple for the Urn of Sacred Ashes. Phantom-Jowan congratulates the Warden on becoming the mage he never could.
In the tower, Jowan cries out in desperation that he started dabbling in blood magic (learned from a book, not a demon) in order to become a better mage and hopefully make a better life for him and Lily. Only Gregoire got evidence and eye-witness testimony that Jowan was practicing blood magic (Irving states this when asked about it in the Origin story).
Jowan is renowned for making monumentally bad decisions, but he has never really implied jealousy of the Warden in the flesh. Only worry about his own lack of Harrowing and apparent lack of skill. And him trying to keep his relationship with Lily a secret. And no, Jowan honestly doesn't seem the type to butcher Lily.
The evidence of his good character is when I let him out of prison, and told him to escape as this was his last chance. He stubbornly refused and stayed to help, knowing he would likely be executed when all was done. He stayed, never used any magic on anyone except in the blood ritual, which Isolde volunteered for.
When a person stays to fight when all that awaits them is death in an unmarked grave or tranquility, that takes remarkable courage and is not the characteristics of a man consumed with power and thoughts of escape.