@BlueCatMage no, his death won't be painless either way. It just... with Jeanette, he will die quicker, therefore, he will end his suffering quicker. With Athy, he will live longer but with the curse attacking him periodically.
My theory is Athy could be the cure for his curse that is why the black magic inside his body trying to reject Athy's presence and made him suffer whenever he was with her. All this time, it could be that Claude had been suppressing the curse in his body and the curse would seep through from Claude and try to harm Athy so when it accumulated enough, she would cough blood and get sick as her mana went rampant trying to fight back the curse. Remember how Lucas would sometimes poke her head to get rid of the remnant of black magic in her body.
Thus when Lucas is gone and Athy got sick, her dad gave his magic to Athy which made his body no longer able to suppress the black in his body. This could be why the curse is trying to resist Athy.
Just a casual theory with a lot of holes. Shall not be taken seriously.